Preheat the oven to 400F. Toss the cauliflower, red onion, and paprika in one tablespoon of the oil. Transfer the vegetables to two parchment or foil-lined baking sheets. Sprinkle generously with coarse salt and crushed red pepper (less generously with the pepper, if you're sensitive to heat). Roast for 20-25 minutes, or until the cauliflower is tender and crispy on the edges, stirring once halfway through.
While the vegetables roast, bring a large pot of salted water to boil. Add the pasta. Cook according to package instructions, stirring frequently, until the pasta is done. Drain, reserving 1/2 cup cooking water.
Heat the remaining tablespoon of oil over low heat in a large saute pan. Add the garlic and cook for about a minute, or until it's fragrant but not burning. When the garlic is ready, add the pasta, the lemon juice and zest, the parsley, and the roasted vegetables. Stir everything together, then add extra salt and an extra dash of crushed pepper to taste. If the pasta feels too thick or is sticking together at all, stir in a splash of the cooking water. Serve right away.