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