Start your sauce base. In your large pot or deep skillet, heat olive oil and butter over medium heat until butter melts. Add minced garlic and red pepper flakes (if using) and cook for 30-60 seconds until fragrant but not burned. If you burn the garlic, there's no coming back – just start over. I've been there, no judgment.
Make it creamy. Sprinkle the flour over the garlic and stir constantly for about 1 minute to cook out the raw flour taste. Nobody wants a sauce that tastes like uncooked biscuits.
Add your liquids. Gradually pour in the broth while whisking constantly to avoid lumps (lumps = sad sauce). Bring to a gentle simmer, then pour in the cream or half-and-half. Let it simmer for 2-3 minutes until it starts to thicken slightly. The sauce will continue to thicken as it cooks with the tortellini.
Cook those pasta pockets. Add the tortellini directly to the simmering liquid. No need to cook them separately – that's what makes this one-pot magic! Stir gently and reduce heat to medium-low. Cover and cook for about 5 minutes or according to package directions. The tortellini will absorb some of the liquid and release starch, helping to thicken the sauce.
Green it up. Once the tortellini is almost tender, add your greens by the handful, stirring until they wilt down. This should only take 1-2 minutes. If using heartier greens like kale, you might need an extra minute.
Finish with flavor boosters. Remove from heat and stir in the Parmesan cheese, lemon zest, and lemon juice. The cheese will melt and further thicken the sauce, while the lemon brightens everything up. Season with salt and pepper to taste.
Serve it up! Divide among bowls, top with fresh herbs, more Parmesan, and a twist of black pepper. Look at you, cooking like an actual adult!