Mini Vegetarian Frittatas

Preheat the oven to 375 degrees F (190 degrees C). Grease 7 muffin cups with cooking spray.

Melt butter in a skillet over medium heat. Add mushrooms, herbes de Provence, salt, and pepper; cook and stir until tender and starting to brown, about 5 minutes. Stir in scallions and cook until softened, 1 to 2 minutes. Add chard and water; cook and stir until wilted, about 2 minutes. Remove from heat and cool for 5 minutes.

Whisk eggs, milk, Parmesan cheese, and Dijon mustard together in a large bowl. Fold in mushroom mixture and sun-dried tomatoes. Pour egg mixture into the prepared muffin cups.

Bake in the preheated oven until puffed and golden brown, about 15 minutes. Cool frittatas for 5 minutes before lifting out of the muffin cups.