Easy Sheet-Pan Eggs with Mushrooms & Spinach

Preheat oven to 300°F. Coat a large rimmed baking sheet with cooking spray.

Heat oil in a large skillet over medium-high heat. Add onion; cook, stirring, until light golden, 4 to 5 minutes. Add garlic; cook, stirring, until fragrant, about 30 seconds. Add mushrooms, thyme and 1/4 teaspoon each salt and pepper; continue cooking and stirring until the vegetables are tender, 5 to 7 minutes more. Stir in spinach and remove from heat.

Whisk eggs, milk, mustard and the remaining 3/4 teaspoon each salt and pepper in a large bowl. Stir in Cheddar. Pour onto the prepared baking sheet, then sprinkle with the vegetable mixture. Bake, rotating the pan from back to front halfway, until just set, 20 to 25 minutes. Cut into 12 squares and serve.