Place a large pot with 4 quarts salted water on stove and heat on high to boiling. When water reaches a boil, carefully drop dried pasta into water, stir, cover pot and turn off heat. Set timer for 7 minutes (or time on package). When pasta is cooked, drain, toss with 1 tablespoon oil and set aside.
Meanwhile, heat 2 tablespoons olive oil over medium heat in large skillet or saucepan with high sides. Add onion and garlic. Cook, stirring occasionally, until onion is softened, 2-3 minutes. Add tomato paste and harissa to pan and stir to combine.
Raise heat to medium-high, add tomatoes and bring sauce to a simmer, stirring occasionally, for 5 minutes. Add shrimp and cook until pink and cooked through, 5-7 minutes.
Add torn basil leaves to the shrimp and sauce. Stir and season with salt and pepper to taste.
Serve shrimp in spicy harissa sauce over pasta, garnished with fresh basil.