Prepare the bread and vegetables. Lightly grease a 9 x 13-inch metal baking pan. Cube and weigh the bread. Drain, dry, and prepare 200 g roasted pepper pieces. Finely dice the onion and chop large spinach leaves. Use a metal pan suitable for refrigerator-to-oven use if preparing overnight; do not plan to put a cold glass dish directly into a hot oven.

Cook out spinach moisture. Heat olive oil in a skillet over medium heat. Cook the onion about 6–8 minutes until softened, then stir in garlic for 30 seconds. Add spinach and cook until wilted and excess moisture has evaporated, about 2–3 minutes. Transfer to a clean bowl and let the mixture cool briefly while preparing the custard.

Whisk the custard and layer. Whisk eggs, milk, oregano, salt, and pepper until evenly combined. Spread half the bread in the pan, scatter over half the cooked vegetables, peppers, and feta, then repeat with the rest. The layering spreads the filling through the bread rather than leaving every piece of feta on the surface.

Pour and soak cold. Pour the custard evenly over the layered ingredients and press the bread gently with a broad spatula. Cover and refrigerate for 30 minutes. For an overnight version, refrigerate 8–12 hours and bake within 24 hours of assembly. Keep the raw custard cold throughout; this resting stage is for absorption, not cooking.

Bake covered, then uncovered. Heat a conventional oven to 350°F (175°C). Bake loosely covered with foil for 25 minutes, then uncover and bake about 20–30 minutes more. The exposed bread should be golden, the center set, and the egg custard at least 160°F (71°C). A cold overnight pan may take longer; tent again if the surface browns too quickly.

Rest and portion. Rest for 10 minutes before dividing into eight portions. Use a wide spatula to lift the soft bread custard with its filling. Serve immediately or refrigerate leftovers promptly in shallow containers. The strata settles as it cools, so do not judge doneness by whether the oven-high puff remains.