Preheat your oven to 325°F.
Place the beef pieces in a large oven-safe braiser or Dutch oven and season well with black pepper.
Add the onion, garlic, thyme, and mushrooms around the meat.
Pour the beef broth over the mixture and cover the pot with a tight-fitting lid.
Transfer the pot to the oven and cook for 2½ to 3 hours, or until the beef becomes very tender and easy to pull apart.
Raise the oven temperature to 400°F.
Remove the lid and skim off any excess fat from the surface.
Add the heavy cream, kale, and butter.
Return the pot to the oven uncovered for 15–20 minutes, allowing the sauce to thicken and the beef to brown slightly.
In a small skillet, melt a small amount of butter.
Add the sage leaves and cook just until crisp and fragrant.
Transfer to a plate to cool.
Serve the braised beef with plenty of creamy mushroom sauce over mashed potatoes, rice, noodles, or warm bread.
Finish each serving with the crispy sage on top.