Bring a large pot of salted water to a boil. Add the beef tortellini and cook according to package instructions until al dente, usually about 3 to 5 minutes. Drain and set aside.
While the tortellini cooks, heat olive oil in a large skillet over medium heat.
Add the chopped onion and cook until softened, about 3 minutes.
Add the minced garlic and cook for an additional 1 minute until fragrant.
Add the ground beef to the skillet and cook, breaking it up with a spoon, until browned and cooked through, about 6 to 8 minutes. Drain excess fat if necessary.
Stir in the diced tomatoes with their juice, tomato sauce, dried Italian seasoning, salt, black pepper, and red pepper flakes.
Bring the sauce to a simmer and cook for 8 to 10 minutes, stirring occasionally, until slightly thickened.
Add the cooked tortellini to the skillet and gently toss to coat in the sauce.
Remove from heat and sprinkle with grated Parmesan cheese and chopped fresh basil before serving.