Preheat the oven to 375℉.
In a large oven-safe skillet, heat the oil over medium-high heat. Add the Italian sausage and the 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 and return to the skillet.
Reduce the heat to medium low and add the chopped kale, cook, stirring, until it has slightly wilted, about 2 minutes.
Add the marinara, broth, lemon zest, lemon juice, salt and pepper and stir until well combined. Bring to a simmer.
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 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.