Heat 1 tablespoon olive oil in a large saucepan over medium heat until hot. Add the 1 small diced onion and cook, stirring occasionally, until softened, about 3 minutes.
Add 2 minced garlic cloves, 1/2 teaspoon dried basil, and 1/4 to 1/2 teaspoon crushed red pepper flakes. Cook, stirring, for 1 minute until fragrant.
Pour in the 1 (24-ounce) jar spaghetti sauce and the 1/2 cup heavy cream. Stir in 1/4 teaspoon black pepper and 1/4 teaspoon salt to combine.
Bring the sauce to a gentle simmer, then reduce heat to low and simmer, uncovered, stirring occasionally, for 10 minutes or until the sauce is thickened to your liking.
While the sauce simmers, cook 1 pound angel hair pasta according to the package directions until al dente. Drain the pasta.
Add the drained pasta to the saucepan and toss gently over low heat until the pasta is evenly coated with the sauce, about 1 minute. Serve immediately.