Preheat the oven to 350°F (175°C).
In a large mixing bowl, combine the ground beef, ground pork, breadcrumbs, 1 cup shredded cheddar cheese, milk, eggs, chopped onion, minced garlic, salt, pepper, oregano, and thyme. Mix until just combined.
In a skillet over medium heat, sauté the carrots, zucchini, and bell peppers for about 5 minutes, until slightly softened. Remove from heat and let cool slightly.
Fold the sautéed vegetables into the meat mixture until evenly distributed.
Transfer the mixture to a greased 9x13 inch baking dish, pressing it down into an even layer.
Spread the marinara sauce evenly over the top of the meat mixture.
Cover the dish with aluminum foil and bake in the preheated oven for 30 minutes.
After 30 minutes, remove the foil and sprinkle additional shredded cheddar cheese on top. Bake uncovered for another 20 minutes, or until the cheese is bubbly and the meatloaf is cooked through (internal temperature should reach 160°F / 70°C).
Let the casserole rest for 10 minutes before slicing and serving.