Preheat oven to 425 degrees F. Place zucchini, mushrooms, onions and garlic on baking sheet. Sprinkle with salt and pepper. Drizzle with olive oil and red pepper flakes to taste. Toss and bake until vegetables are tender and caramelized, about 20 minutes. Let cool. Process into a puree.
Whisk together the eggs and herbs in a large bowl. Add the meat, steak seasoning, breadcrumbs, cheese, 1/2 cup of the ketchup, 2 Tbsp. of the balsamic vinegar, and the cooled vegetable puree and mix until just combined.
Mold the meatloaf on a baking sheet. Whisk together the remaining ketchup, balsamic vinegar, and red pepper flakes in a small bowl. Brush the mixture over the entire loaf. Bake for 1 to 1-1/4 hours. Let rest 10 minutes before slicing.