Saute onion with water or dry for 3-5 minutes until lightly brown. Add the chili powder, stirring to coat.

Transfer the onion mixture to your slow cooker, if needed. Add the Mung beans, tomatoes, water, soy sauce, mustard, sweetener, and black pepper. Stir to combine. Cover and cook on low for 8 hours or on high for 4 hours.

Serve on toasted buns or English muffins. These can also be served in bowls.