Heat the oil in a Dutch oven over medium heat for 1 minute.
Stir in the onion and season with salt. Cook, stirring frequently, until softened and translucent, about 8 minutes.
Add the tortellini and water, then quickly cover with a lid. Let the tortellini steam for 5 minutes.
Uncover the Dutch oven and stir in the cream, tomatoes, and mustard.
Reduce the heat to low and add the cheese, stirring constantly until the cheese is melted, everything is combined, and the sauce is thick, about 3 minutes. Season with salt and pepper, to taste.
Notes
Cook it over a campfire--To make this over a campfire, set your Dutch oven above a bed of hot coals. Depending on the distance from the coals, the cooking times may be slightly different.
Be careful!--Use caution when removing the lid of your Dutch oven after steaming the tortellini as there may be a rush of hot steam.
Storage--Store leftover pasta in a sealed container in your cooler or fridge for up to 3 days.