Overnight Breakfast Casserole You Bake Straight From Cold

• A metal dish tolerates the planned refrigerator-to-oven move and conducts heat efficiently into the cold center.
• Cooking onion, spinach, and sausage until dry prevents their overnight liquid from collecting beneath the bread and eggs.
• Day-old sourdough provides structure during the long soak, so portions remain sliceable rather than settling into a heavy uniform layer.
• The method identifies the center checkpoint explicitly, because browned edges alone cannot tell you whether the deepest portion has reached the intended texture.

Ingredient notes

• Sourdough cubes: The specified weight controls how much custard or filling the dish can hold. Use day-old bread when listed, and keep frozen potatoes frozen when called for; those details protect the intended structure.
• Eggs: Whisk until no distinct whites remain. Uniform eggs set evenly around the other ingredients, while under-mixed whites can leave rubbery streaks beside softer custard. The listed large eggs weigh about 50 g each without shells.
• Milk and dairy: Whole milk supplies moisture without making the mixture thin. Measure by weight for repeatable results. Cheese or cream adds richness, but increasing it casually can delay the center and make the casserole feel heavy.
• Seasoning: Salt the liquid mixture, where it disperses through the entire pan. The measured spice should support the eggs rather than bury their flavor.
• Pan choice: a metal 9-by-13-inch baking dish gives the stated depth and timing. Grease the corners carefully, because that is where egg mixtures attach first and where a clean serving is hardest to lift.

1. 1Prepare the foundation

   Cook onion in oil until soft. Add sausage and spinach, then continue until the spinach is wilted and the skillet looks dry. Spread sourdough in a greased metal dish and distribute the sausage mixture and cheddar between the cubes. Keep the components loose enough for custard to pass through the gaps. Keep the layer level so the pan cooks at the same pace from edge to center.

2. 2Build the decisive layers

   Whisk eggs, milk, dry mustard, salt, and pepper until completely blended. Pour slowly over the dish, working from edges to center. Press the bread once, release it, and cover. Refrigerate for 8 hours. Before baking, confirm the bread looks hydrated and no watery vegetable liquid has collected around the perimeter.

   

3. 3Bake toward the center cue

   Uncover and move the cold metal dish directly into a 350°F (175°C) oven. Bake for 45 to 50 minutes. The edges should rise slightly away from the sides, and the center should hold a gentle set when nudged. A thin knife inserted into the middle should show tender crumbs without liquid egg.

4. 4Check texture and portion

   Leave the casserole undisturbed briefly before cutting 8 squares. Run a flexible knife around the perimeter, then lift portions with a broad spatula so the soaked bread and savory filling remain layered. Scatter a few scallion slices over the top. Avoid covering the hot pan, which can make the upper bread soft. Notice both the exposed top and the lowest layer as you lift the first piece.

   

• Weigh the main bread or potato component. Volume shifts when pieces are packed, and the liquid ratio depends on the listed amount.
• Distribute solid fillings to every corner before adding eggs. Rearranging them afterward disturbs the layers and creates heavy patches.
• Judge the center, not the browned rim. Edge portions firm first, so the middle provides the useful doneness cue.
• Keep the dish uncovered during baking unless the surface is coloring far ahead of the center. Covering softens carefully exposed edges.
• Use a thin knife for checking and a broad spatula for serving. That combination preserves the layers instead of compressing them.

Serve squares with tomato wedges, citrus, or a bitter green salad. Hot sauce belongs at the table so every portion can be adjusted without soaking the casserole. On a Tuesday, refrigerate pre-cut pieces in shallow containers; the cold-to-oven work is already done, and breakfast only needs reheating. Portion from both the center and edge over successive meals so one container does not collect only firmer corner pieces.