Heat the oil in a large pot over medium-high heat. If using, add the mustard seeds and cook for 1-2 minutes or until they begin to pop. First, add the red onion and ginger and cook until beginning to brown and soften, about 5 minutes. Next, add the cumin, turmeric, cayenne powder, and garlic and cook for 45 seconds until fragrant. Finally, add the sweet peppers and reduce heat to medium and cook for about 5 minutes until softened.