Cut your vegetables into strips, with the onions thinnest, bell pepper next, and the Portobello thickest.
Over high heat, heat your large pan first [note#1], then throw in raw onions and keep them moving until they've softened and started to brown, about 5 minutes [note#2].
Next add your bell pepper and cook for an additional 2 minutes or so before adding your Portobellos and cooking for an additional 4 minutes or so, until everything has just started to soften.
Add chili powder, pepper, and any other desired spices and cook for an additional 1-2 minutes.
Sprinkle with lime juice, toss one more time, and turn off the heat.
Serve right away on a corn tortilla with beans and guacamole, or over rice. You can also save it and store it for future meals.
Enjoy!