Place the chicken breasts in a saucepan with tomatoes (halved), onions, garlic, and salt. Cover with water and cook for 35 minutes.
Remove chicken, shred, and set aside.
Make chipotle sauce by blending tomatoes, onion, garlic, chipotle chillies, and some chicken stock until smooth.
Shred chicken and chop slightly. Slice the other onion.
Heat oil in a saucepan, fry sliced onion, add chipotle sauce and shredded chicken. Cook for 10 minutes.
Serve over tostadas and garnish with feta cheese, sour cream, and avocado slices.
