Place a large pot on the stove over medium-high heat.Once the pan has heated up add in the olive oil and swirl the pan so that the entire surface is coated.Add the meatballs in a single layer and brown for about 2 minutes per side. If you prefer, you can skip this step but I like the color and texture it provides for the meatballs.Once the meatballs are browned, remove them from the pot onto a plate and set aside.Add butter, mushrooms, and onions to the pot and sautee until mushrooms soften and onions start turning translucent, about 3 - 5 minutes.Add garlic and stir for about 1 minute.Season ingredients with a pinch of salt and pepper, then add in tomato paste. Stir while continuing to cook for about 1 minute.Add in beef broth, tomatoes, oregano, and red pepper flakes. Stir to break up any large chunks of tomato and to thoroughly combine.Bring soup to low boil and then add in meatballs and pasta. Cook for about 6 - 8 minutes or until pasta is very al dente.Add in vegetables and simmer until hot and pasta is at desired doness. Taste for seasoning before serving.According to the pasta and veggies you used, the soup may have thickened more than you want. If it's too thick, just add in more beef broth until the desired consistency and heat through.Garnish with herbs and cheese before serving if desired.