Preheat the oven to 175C (350F). Place the red onions, bell peppers and cherry tomatoes on a baking sheet, drizzle with olive oil and bake in the oven for 15 minutes until the peppers are tender.
In a bowl whisk together the eggs, almond milk, mustard, salt, pepper, parsley and oregano.
In a greased baking dish approx 9 x 5 inches in size (8 cups) evenly layer the peppers, red onion, tomatoes, olives, spinach and feta (if you are using it). Pour in the egg into the dish and place the baking dish in the oven to bake for 30 minutes until the egg has completely set and the casserole is firm to the touch.