Cook pasta according to package instructions, reserving ½ cup of pasta water.
Heat the oil in a large pan over medium-high heat. Sauté onion and garlic until soft, about 10 minutes. Add roasted peppers and sauté until heated through. Remove from heat and let cool slightly.
Place mixture in the bowl of a food processor with stock and all but 2 tablespoons of the feta. Process until combined and smooth, about 30 seconds.
In a pot on low heat, toss pasta with sauce, adding pasta water by the tablespoon, if needed.. Season with salt and pepper, to taste. Divide among pasta bowls. Sprinkle with parsley and remaining feta cheese.