Preheat a large saucepan or Dutch-oven over moderate heat and add the oil. Sweat the onions in the oil to soften. Add the garlic, red pepper flakes, cooking until aromatic 20-30 seconds. Stir in the tomato paste and cook to darken slightly. Deglaze the pan with vodka, simmering to reduce until almost dry.
Add the tomatoes, stock and cashews. Stir and bring to a boil, then reduce to a simmer. Cover and cook for 15 minutes. Use a stand or immersion blender to blend smooth.
Add fettuccini noodles to a separate pot of lightly salted boiling water. Cook the pasta al dente, then remove from the water, reserve the liquid.
Pour the Prepared sauce over the noodles, adjusting sauce consistency as needed with the hot reserved liquid. Season with salt to your tastes. Garnish with green olives and serve warm.