Spinach, Mushroom & Egg Casserole

Ingredients

• 1 teaspoon salt
• 1 teaspoon onion powder
• 2 teaspoons dry mustard
• 1 pound cremini mushrooms, trimmed and sliced
• 2 tablespoons extra-virgin olive oil
• 5 ounces baby spinach
• 12 large eggs
• ¾ cup half-and-half
• 2 teaspoons fresh thyme leaves
• 1 ½ cups shredded Gruyère cheese, preferably cave-aged , divided
• 5 medium cloves garlic, finely chopped