Cook onions, bell peppers, anchovy paste, salt, and pepper in oil in a 4- to 6-quart heavy pot over moderately high heat, covered, stirring occasionally, until vegetables begin to soften, 10 to 15 minutes. Remove lid and cook, stirring occasionally, until onions are golden, 10 to 15 minutes more. While onions are browning, cook rotini in a 6- to 8-quart pot of boiling salted water until al dente. Reserve 1 cup pasta cooking water, then drain pasta in a colander. Cook pasta with onion mixture in pot over high heat, tossing and adding enough reserved cooking water to keep pasta moist, 1 minute. Transfer pasta to a serving bowl and toss with dill, two thirds of feta, and salt and pepper to taste. Sprinkle remaining feta on top.