Egg, spinach and feta strata - chinese grandma

Ingredients

• 1 pound fresh spinach (or 10 ounces frozen; whole or chopped)
• 1-2 tablespoons olive oil
• Salt and pepper for seasoning spinach
• 3 slices bread, cubed (can use more for thicker casserole; this amount will give you a more custardy texture)
• 1 1/2 cups cheddar cheese, grated
• 1/2 cup crumbled feta cheese
• 9 eggs
• 3 cups milk
• 1 1/2 teaspoon ground mustard powder, mixed with 1 teaspoon water
• 1 teaspoon salt
• 1/2 teaspoon pepper, or to taste