In a hot skillet add olive oil and cook chicken for 6 minutes on each side, or until cooked through. Season with salt and pepper and set aside to cool.
Melt butter in a large pot over medium-high heat. Sauté onion until translucent (about 2 min). Add garlic and cook until fragrant. Add flour and whisk 2-3 minutes, until golden. Add chicken broth and half/half. Let simmer 15 minutes, until thickened.
Add tortellini to pot and cook 10 minutes, until al dente. Add diced chicken and season with crushed red pepper flakes, salt, and pepper. Garnish with basil. Serve warm.