Heat oil in a large skillet over medium heat; cook and stir sausage until browned, about 5 minutes. Add onion to sausage; cook and stir until slightly browned, about 5 minutes. Stir sweet pepper, oregano, thyme, cayenne pepper, celery salt, and red pepper flakes into sausage mixture; cook and stir until fragrant, 1 to 2 minutes.
Mix quinoa into sausage mixture; cook and stir until quinoa is slightly toasted, about 1 minute. Pour broth and marinara sauce over quinoa mixture. Cover skillet and simmer until quinoa is tender, about 15 minutes.