Step 1: Pat the boneless, skinless chicken thighs dry with paper towels. Season generously on both sides with salt, black pepper, and paprika.
Step 2: In a large skillet or Dutch oven, heat 1 tablespoon of olive oil over medium-high heat. Once shimmering, add the seasoned chicken thighs. Sear for 4-5 minutes per side, until golden brown and cooked through. Remove the chicken from the skillet and set aside on a cutting board to rest.
Step 3: While the chicken rests, reduce the heat to medium. Add the butter to the same skillet, scraping up any browned bits from the chicken. Once melted, add the minced garlic, finely chopped shallots, and red pepper flakes. Sauté for 1-2 minutes until fragrant, being careful not to burn the garlic.
Step 4: Stir in the chicken broth and heavy cream, bringing the mixture to a gentle simmer. Add the fresh lemon juice, Dijon mustard, and all the fresh herbs (parsley, chives, thyme). Season the sauce with a little more salt and pepper to taste. Let the sauce simmer gently for 5 minutes, allowing it to slightly thicken and the flavors to meld.
Step 5: While the sauce simmers, cook the tortellini according to package directions in a separate pot of salted boiling water until al dente. Before draining, reserve about 1/2 cup of the pasta water.
Step 6: Cut the rested chicken thighs into bite-sized pieces. Add the cooked tortellini and chicken pieces to the skillet with the cowboy butter sauce. Add the fresh spinach and stir everything together, tossing to coat the tortellini and chicken evenly. If the sauce seems too thick, add a tablespoon or two of the reserved pasta water until it reaches your desired consistency.
Step 7: Continue to cook for another 2-3 minutes, or until the spinach has wilted and the tortellini and chicken are thoroughly heated through and coated in the glorious cowboy butter sauce.
Step 8: Serve immediately, garnished with extra fresh parsley or grated Parmesan cheese, if desired.