Bring a large pot of salted water to a boil. Add the pasta and cook according to package directions until al dente. Just before draining, remove 1 1/2 cups of the pasta cooking water. Drain.Heat 2 tablespoons olive oil in a large skillet over high heat. When the oil shimmers, add the chicken sausage and brown all over, about 5 to 8 minutes. Reduce the heat to low, add the shallots, garlic, oregano, thyme and a pinch of red pepper flakes. Cook until the shallots begin to caramelize, about 5 minutes. Stir in the tomato paste, cook 2 minutes. Stir in the crushed tomatoes, 1 cup pasta cooking water, and season with salt and pepper. Simmer for 10-15 minutes or until the sauce thickens slightly. Add the greens, stirring until wilted. Taste and adjust seasonings as needed.Add the pasta shells and 1/2 cup basil, tossing to combine. If needed, thin the sauce with additional pasta cooking water.In a small bowl, mix the remaining 1/4 cup olive oil with the remaining 1/2 cup basil, lemon zest, lemon juice, and a pinch of salt. Spread the ricotta cheese into the bottom of each pasta bowl. Spoon the shells and sauce over the ricotta. Drizzle the basil lemon sauce overtop. Serve and enjoy!