In a large skillet over medium heat, combine onion, oil, 1/4 tsp. salt and crushed red pepper. Cook, stirring occasionally, until the onion is a rich golden color, 10 to 15 minutes.
Meanwhile, peel bell peppers with a vegetable peeler. Cut them in half, remove the cores and thinly slice into 1/4-inch-wide strips.
When the onion is golden, increase heat to medium-high and add the peppers. Cook, stirring occasionally, for 2 minutes. Reduce heat to medium; add tomatoes and 1/4 tsp. salt and cook until the tomatoes are no longer watery and have separated from the oil, 15 to 20 minutes. Add parsley, stir for 30 seconds, then remove from heat.
Meanwhile, bring 2 qts. of water to a boil in a large pot. Add the remaining 1 Tbsp. salt, then add pasta and stir well. Cook according to package instructions until just tender. Drain well and toss with the sauce. Serve at once.