Preheat oven to 450ºF. Lightly grease 8 muffin cups or place a foil liner in each.
In a medium saucepan, warm oil over medium-high heat and sauté onion until softened and translucent, 3 to 5 minutes. Stir in tomatoes, beans and chili powder and bring to a boil. Reduce heat and simmer for 5 minutes, until chili is slightly thickened. Season with salt.
While chili is cooking, combine biscuit mix and milk in a medium bowl. Turn out onto a lightly floured work surface and pat into a 6-by-9-inch rectangle. Cut into 8 squares and put each square into a muffin-tin cup, pressing down gently and pushing slightly up sides of each cup.
Scoop about 1/4 cup chili into each cup. Sprinkle each with cheese. Bake until chili and cheese are bubbling, about 10 minutes.