Heat the oil in a heavy skillet over medium-high heat. Saute onion and garlic until soft, about 10 minutes. Add roasted peppers and saute 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. Cook pasta according to package directions. Drain, reserving 1/2 cup pasta water. Toss pasta with sauce, adding pasta water by the tablespoon, if needed. Sauce should cling nicely to pasta. Season with salt and pepper, to taste. Divide among pasta bowls. Sprinkle with parsley and remaining feta cheese.