Halve the spaghetti squash lengthwise and remove the seeds. Put the squash in a large microwave-safe bowl with 2 tablespoons water. Cover with plastic wrap and poke a hole in the plastic to vent. Microwave until just tender, 15 to 17 minutes. Uncover and let cool slightly. Scrape the flesh into strands with a fork. Transfer to a colander and season with salt and pepper. Let sit in the colander.
Meanwhile, heat the olive oil in a large nonstick skillet over medium-high heat. Add the onion, garlic and a big pinch of salt; cook, stirring frequently, until the onion is softened, about 5 minutes. Increase the heat to high and add the beef; season with salt and pepper. Cook, breaking up the meat with a wooden spoon, until no longer pink, 3 to 5 minutes. Stir in the tomato paste and cumin; cook until slightly darkened, about 2 minutes. Add the tomatoes, raisins and 1/2 cup water. Bring to a boil, then reduce the heat to a simmer and cook until the sauce is thickened, about 8 minutes. Stir in the olives and half the cilantro. Season with salt and pepper.
Press down gently on the spaghetti squash to remove any excess liquid, then divide among plates. Top with the beef mixture and the remaining cilantro.