In a large skillet, over medium high heat, cook your onion until translucent, about 3 minutes.
Next add the ground beef, and using a wooden spoon, break up the meat until its totally browned, then drain the fat from the pan.
Add the spices to the meat and stir to combine. Add 1/4 cup hot water and stir. Reduce heat to low and simmer for 10 minutes.
Assembly:
Drizzle the portobello caps with the olive oil and season both sides with salt and pepper to taste. Heat a grill pan on high and cook the caps 5-6 minutes per side or until fork tender (some caps are thicker than others).
Evenly divide the beef taco mixture between the caps and top with the cheddar. Cover with a lid until the cheese melts. Serve with salted tomato and avocado and garnish with chopped fresh cilantro.
Notes
You can also sub in ground turkey, ground chicken or sausage.