Preheat an oven to 350°F.
Place a sauté pan over medium heat and wait until it’s hot. Add the oil to the pan and then add the ribs, fat-cap-side down. Sauté for about 1 minute until the fat is golden brown.
Turn the ribs and continue sautéing, turning as necessary so that all sides are golden brown, for a total sauté time of about 5 minutes.
Transfer the ribs to a heat-proof casserole, fat-cap-side-up. About 3 Tbsp fat should remain in the sauté pan; if there’s more, drain some off.
Add the onions to the pan and sauté over medium heat for about 1 minute until golden brown. Add the garlic, and then stir in the step 3 chili powder and flour to make a roux. Cook, stirring constantly, for about a minute longer until the roux acquires a rich brown color.
Slowly stir in the beer, followed by the stock, to make a smooth sauce.
When the sauce comes to the simmer, stir in the mustard, brown sugar, Worcestershire sauce, bay leaf, sage, and thyme. Season the sauce lightly with salt, keeping in mind that it will reduce and concentrate flavor during braising.
Pour the sauce over the ribs and place the uncovered casserole in the oven. Bake for about 30 minutes. Baste once or twice.
Turn over the ribs so the bone sides face up. Continue baking for about 10 minutes or longer, until a knife pierces the meat with little resistance.
If prepping ahead, cool the casserole to room temperature. Transfer the ribs and their sauce into a container and refrigerate.