Cook spaghetti according to instructions in a large pot. Drain, reserving 1 cup of pasta water. Set aside.
In the same pot, melt the butter over medium heat. Add garlic, salt and pepper. Stir for 1 minute or until garlic is fragrant.
Add the spaghetti back into the pot with the lemon juice and lemon zest. Stir in 3/4 cup of reserved pasta water and 1/2 cup parmesan cheese. Toss to coat. Add more pasta water if needed.
Slice and cook the chicken sausages in a skillet over medium heat until they are heated through.
Remove the lemon pasta from the pot. Add the vegetable oil to the same pot. Add grape tomatoes and cover with a lid. Turn the heat to medium-high and cook the tomatoes for 3–4 minutes, occasionally stirring until they darken in color.
Top the pasta with the sausages, tomatoes and remaining parmesan cheese.