01 - Preheat oven to 400°F. Line baking sheet with parchment. Toss cherry tomatoes with 1 tbsp olive oil, pinch of salt, and pepper. Spread cut side up and roast 15 minutes until softened and caramelized. Set aside.
02 - Reduce oven temperature to 350°F. Grease 9x13-inch baking dish with butter or cooking spray.
03 - Heat remaining 1 tbsp olive oil in skillet over medium heat. Sauté onion 3-4 minutes until translucent. Add spinach and cook until wilted, about 2 minutes. Remove from heat.
04 - In large bowl, whisk together eggs, milk, cream, Dijon mustard, garlic powder, salt, and black pepper until fully combined.
05 - Add bread cubes to custard mixture. Toss gently to coat evenly. Stir in sautéed onion, spinach, cheddar, and Gruyère.
06 - Pour mixture into prepared baking dish. Arrange roasted cherry tomatoes evenly over top.
07 - Cover tightly with aluminum foil. Bake 30 minutes to allow bread to absorb custard.
08 - Remove foil and continue baking 20-25 minutes until golden brown and set in center. A knife inserted should come out clean.
09 - Let strata cool 10 minutes before serving. Sprinkle with fresh chives or parsley. Serve warm.