Preheat the oven to 375℉.
Heat the oil in a large oven-safe skillet over medium-high heat. Add the sausage and onion and cook, breaking up the meat with the edge of a wooden spoon, until cooked through and no longer pink, 5 to 7 minutes. Drain off excess fat. any
Reduce the heat to medium-low. Add the kale and cook, stirring, until it has slightly wilted, about 2 minutes.
Add the marinara sauce, broth, lemon zest, lemon juice, salt, and pepper and stir until well combined. Bring to a simmer, about 5 minutes.
Once simmering, remove from the heat and add the tortellini. Gently stir to combine and spread into an even layer in the skillet. Top with the mozzarella cheese. Transfer to the oven and bake until the tortellini is tender and the cheese is golden brown, about 20 minutes.
Remove from the oven and let cool for 5 minutes. Garnish with fresh basil and serve with crushed red pepper flakes, if desired.