Start by transferring the ground beef and the finely chopped crimini mushrooms into a large mixing bowl. Make sure the mushrooms are really chopped finely. It’s crucial because you want them to blend in with the beef, avoiding any funky mushroom chunks. Sprinkle in the taco seasoning, dried parsley, kosher salt, cracked black pepper, and garlic powder. Use your hands to gently combine all these ingredients, ensuring everything is well mixed, but don’t overwork it. Next, form the mixture into two patties, about ½ inch thick.

Heat your grill or skillet over medium heat. Once it's nice and hot, place the patties onto the grill. Cook them for about 4-6 minutes on each side for medium doneness. When there’s about a minute left in cooking, lay the slices of pepperjack cheese on top of each patty. Cover with a lid if you have one; this helps the cheese melt beautifully.

While the burgers are grilling, heat a separate skillet over medium-high heat and add a drizzle of olive oil for good measure. Toss in the sliced red and yellow bell peppers along with the yellow onion. Sauté them fajita-style until they are soft and slightly caramelized, about 4-5 minutes. This not only adds flavor but also brings a lovely sweetness to the dish.

Once everything is cooked to perfection, it's time to build your burgers. Start by toasting your burger buns if you haven’t already. Place each cheeseburger patty on its respective bun, then generously top with the sautéed veggies. Finally, add a serving of the smashed avocado, seasoned with a sprinkle of salt and pepper, for that creamy finish. Close the bun, dig in, and enjoy every delicious bite!