Set a large pot to boil with about 2 tablespoon of salt.
As the water is coming to boil: heat oil in a separate large saute pan or braiser over medium-high heat, then season the diced chicken thighs and cook them. Don't overcrowd the pan, we want the chicken to gain brown color and get flavorful. Cook for 5 minutes, until the diced chicken is fully cooked, then remove from the pan onto a plate. Set aside.
Turn the heat down to medium under the braiser pan, and melt in the butter. Add the minced garlic, lemon zest, and red pepper flakes. Stir it around a little bit and sauté for about 2 minutes, or until the garlic is really fragrant. Don't let it burn!
Pour in the heavy cream and whisk to really get the garlic off of the bottom of the pan. Whisking to make sure the cream doesn't scorch at the bottom, bring the cream to a simmer for about 5-7 minutes, thickening it up. Around this time, your pot of water should be boiling. Add the tortellini pasta to the boiling water and cook til al dente.
Once the cream is thickened, turn the heat to low and add the parmesan cheese. Whisk it in well until fully incorporated. Give the sauce a taste and season as needed with kosher salt and black pepper.
Add the cooked chicken and cooked tortellini pasta to the sauce, tossing gently. The tortellini will absorb some of the sauce, which is why we didn't want to overcook it in the pot.
Serve hot with fresh herbs and parmesan cheese as garnish!