Place chicken, onion, tomato sauce, chipotle, adobo sauce, cumin and salt in a 5-quart slow cooker. Cover and cook on low for 5 to 6 hours until chicken reaches 165°F and pulls apart easily.
Lift chicken out and shred it coarsely. Drain off excess liquid, then mix chicken with refried black beans, Monterey Jack and no more than 1/4 cup of the cooking sauce so the filling holds together without running.
Heat the oven to 425°F and lightly oil a rimmed sheet pan with 1 teaspoon of the measured oil. Warm tortillas in the microwave under a barely damp paper towel for 20 to 30 seconds so they fold without cracking.
Divide filling evenly among tortillas. Fold the sides inward, roll tightly into closed packets and place seam-side down on the sheet pan with space between them.
Brush packets with remaining oil and bake for 20 to 25 minutes, turning carefully once, until golden and crisp and the filling reaches 165°F.
Rest for 5 minutes, then serve one chimichanga per person with 1 tablespoon sour cream alongside.