Preheat the Oven: Preheat your oven to 350°F (175°C). Grease or line a large baking dish (about 9x13 inches) with parchment paper or nonstick spray.

Cook the Vegetables: In a large skillet, warm the olive oil over medium heat. Then, add the diced onion, bell pepper, and broccoli. Sauté for about 5-7 minutes, until the vegetables have softened. Add the halved cherry tomatoes and sauté for another 2-3 minutes. Remove from heat.

Prepare the Batter: In a large mixing bowl, whisk together the flour, milk, eggs, mustard, salt, and pepper until smooth and well-combined.

Mix in the Veggies and Cheese: Add the sautéed vegetables to the batter. Stir in the grated cheese, crumbled feta, and mixed dried herbs (if using).

Add the Pesto: Spoon dollops of pesto across the top of the frittata mixture. You can gently swirl the pesto into the mixture using a knife or leave it as is for bursts of flavor.

Bake the Frittata: Bake the frittata for 30-35 minutes, or until the top is golden and the center is set. You can check for doneness by inserting a toothpick or knife into the center—it should come out clean.

Cool and Serve: Allow the frittata to cool for 5-10 minutes before slicing and serving. Garnish with fresh herbs or an extra sprinkle of cheese, if desired. Serve warm or at room temperature.