Make Meatballs: Combine quinoa and water in a pot and simmer for 10 minutes, or until tender. Meanwhile, add cauliflower to a large pot of water and simmer until fork-tender, about 10 minutes. Strain both and add to a food processor. Blitz a few times until mostly broken down. Add breadcrumbs, eggs, garlic, and seasonings, stirring to combine. Roll into tablespoon-sized balls.
Cook Meatballs: Heat a dash of oil in a large skillet over medium/high heat. Fill the skillet with a batch of meatballs and cook, flipping them often to evenly brown all sides (about 5 minutes per batch). Continue heating new oil and cooking batches until meatballs are all cooked. Place on a paper towel when finished.
Make Sauce: Melt butter in a skillet over medium heat. Whisk in flour and cook until tan, 3-5 minutes. Whisk in 1.5 cups broth (it should thicken quickly!) Add more broth until you reach a thickness to your liking. Allow sauce to cool slightly, then add yogurt or sour cream and season with salt and pepper.