Heat oil in a large skillet over medium-high heat and saute onions until soft and golden. Add the cut-up cauliflower and potatoes, and stir. Put in the turmeric, chili powder, cumin, salt, and tomatoes; stir and fry the mixture for 3 to 4 minutes. Reduce heat to medium-low, cover, and cook until the potatoes and cauliflower are almost tender (about 15 minutes), stirring from time to time to keep the vegetables from sticking. Add the peas, and cook, covered, an additional 5 to 7 minutes. Sprinkle with garam masala and stir gently to mix.