Add chicken breast to the bottom of a slow cooker or Instant Pot, then top with chipotles in adobo and salsa. If slow cooking, cook on low for 7-8 hours. If using Instant Pot, pressure cook on high for 12 minutes (15 minutes if cooking a double batch) followed by a natural release. Chicken should shred easily with a fork when done.
Remove chicken from slow cooker or Instant Pot and shred with two forks. Return chicken to slow cooker with heat off and set aside.
Pre-heat oven to 400 degrees. Line a baking sheet with SILPAT or aluminum foil.
Remove caps from mushrooms. Use a small spoon to scoop out some space for chicken, if needed.
Rub mushrooms with a small amount of olive oil and arrange them on the baking sheet, cap side up. Season with salt, pepper, and garlic powder.
Add shredded chicken to mushrooms, compacting it slightly. Bake at 400 degrees for 20-25 minutes or until chicken begins to brown on the edges.
Top stuffed mushrooms with guacamole, red onion, and cilantro just before serving.