01 - Preheat oven to 400°F. Line baking sheet with parchment. Toss cherry tomatoes with 1 tablespoon olive oil, pinch of salt, and pepper. Spread cut side up and roast 15 minutes until softened and caramelized.
02 - Reduce oven temperature to 350°F. Grease 9x13-inch baking dish with butter or cooking spray.
03 - Heat remaining 1 tablespoon olive oil in skillet over medium heat. Sauté onion 3-4 minutes until translucent. Add spinach and cook until wilted, about 2 minutes.
04 - Whisk together eggs, milk, cream, Dijon mustard, garlic powder, salt, and black pepper in large bowl until fully combined.
05 - Add bread cubes to custard mixture, toss gently to coat. Stir in sautéed vegetables, cheddar, and Gruyère. Pour into prepared baking dish. Arrange roasted tomatoes evenly over top.
06 - Cover with foil and bake 30 minutes. Remove foil and bake additional 20-25 minutes until golden brown and set throughout.
07 - Let cool 10 minutes before serving. Sprinkle with fresh chives or parsley and serve warm.