Preheat oven to 190°C (375°F). Grease a 23 x 33 cm (9 × 13 in.) baking dish. 

Heat oil in a skillet over medium-high heat. Add onion, mushrooms, and sweet pepper and cook until all liquid from mushrooms has evaporated, about 8 minutes. Add spinach and cook until wilted. Let cool slightly. 

In a large bowl, stir together bread cubes, vegetable mixture, and feta. Spread mixture in baking dish. 

In a bowl, whisk together eggs, milk, and seasoning. Pour over bread mixture and let stand for 1 hour or an entire day.

Bake strata on the bottom rack of oven until puffed, golden and set, 40 to 45 minutes.