Melt butter in a large soup pot over medium heat. Add shallots (or onions) then season with salt and pepper and saute until tender, 3-4 minutes. Add garlic then saute for 30 more seconds. Add chicken broth, marinara sauce, and fresh basil then turn heat to high and bring to a boil. Carefully add tortellini to boiling soup then place a lid on top, turn heat down to medium, and then cook until tender. Serve with crostini.