Heat 10-12 in saute pan over medium heat. When hot, add oil or stock to heat. When oil/stock is hot, add onions. Cook for about 10 minutes, stirring occassionally. They will get a little translucent and golden in color.
Add garlic. When fragrant. Add cumin. Stir for about 3-4 minutes until cumin has turned a darker shade of brown.
Add tomatoes and stir into onions to incorporate. With the back of a wooden spoon, gently press down on the tomatoes so they release their juice. This creates a little saucy effect.
Add mushrooms and saute until soft, about 3 minutes.