1. Cook fresh spinach tortellini according to the package directions. Drain well.
2. While the pasta cooks, melt unsalted butter in a large nonstick skillet over medium heat. Add onion and garlic. Cook, stirring often, 1 minute. Add the flour and stir constantly, 30 seconds. Gradually stir in milk, then add the peas and basil. Cook over medium-high heat, stirring constantly, until the mixture simmers and thickens slightly, 3 minutes. Remove the pan from heat, and stir in Parmesan cheese, salt, and pepper.
3. In a large shallow bowl, toss the cooked spinach tortellini and the sauce. Serve warm.
Quick tip: For an easy, earthy spin on this recipe, substitute 1 cup sliced mushrooms for the onion.