Preheat oven to 400°F. Grease a ceramic baking dish with 1 tablespoon of olive oil.
Cut the day-old bread into large pieces and place them in the baking dish. Drizzle 2 tablespoons of olive oil over the bread.
Peel the onion and garlic cloves. Slice the onion into thin rings and mince the garlic. Slice the bell peppers into thin rings.
Heat 2 tablespoons of olive oil in a pan. Sauté the onions, garlic, and bell peppers for 2–3 minutes. Add the cherry tomatoes with their juices, stir to combine, and let the sauce simmer for 5 minutes. Season with salt, pepper, and thyme.
Spread the tomato mixture evenly over the bread. Top with olives.
Cut the feta into thick slices and place on top of the casserole.
Bake the casserole for about 25 minutes, until the feta is lightly browned.