Mince the garlic and roughly chop the remaining vegetables.
In a nonstick pan, melt the 1 tbsp. butter. Sauté the onion, garlic, and peppers until almost soft (~5 minutes), then add the zucchini, yellow squash, and mushrooms. Continue sautéing until all veggies are cooked to your liking. Remove from heat and set aside.
If you have a Panini press, preheat. If not, clean out the pan you sautéed the veggies in and heat over medium.
Mix the yogurt with the pesto. Taste and adjust the ratio if it is too tangy for your liking.
Spread the outsides of the bread with a small amount of butter. On the unbuttered side of 4 slices, put ¼ of the veggie mixture and top ¼ of the mozzarella. Spread the remaining slices of bread with the pesto mixture (on the unbuttered side) and place on top of the veggies and cheese, pesto side down.
Carefully place the sandwiches on the pan or in the Panini maker. If using the Panini press, carefully press down and cook for 5 minutes, until the outsides are crispy and the cheese is melty. If cooking in the pan, press the top down with the back of a spatula and flip after 3-4 minutes, when the bread begins to get crispy, and cook until the other side is done.
Slice in half and serve with warm marinara, if desired.
Notes
I found that using a Panini press allowed me to use all of the veggies if I carefully pressed. If cooking in a pan and flipping, you may have veggie mixture left over. Using gluten free bread will make the sandwich gluten free.
Nutrition Information
Serving size: 1 sandwich Calories: 300
Recipe by Hidden Fruits and Veggies at https://www.hiddenfruitsandveggies.com/2014/03/09/mozzarella-pesto-veggie-melt/