Step 1

Cut your 1/2 rack of spare ribs into 2 or 3 pieces so all the ribs will fit in the pressure cooker. Sprinkle the ribs on both sides with the kosher salt and season with pepper. Pour the beef stock into the pressure cooker and place the steamer insert in the cooker. Place the ribs on the steamer insert. Rather than stacking sections of ribs, place the rib sections on end and sort of curl them around the pot so the pieces aren’t touching to ensure even cooking.

Step 2

Lock the lid in place and bring the pot to high pressure (15 psi for stove top or 9 to 11 psi for electric). If using a stove-top pressure cooker, maintain the pressure for 20 minutes for tender ribs or, if you prefer that the ribs be falling-off-the-bone tender, cook for 30 minutes, adjusting the burner as necessary.If using an electric pressure cooker, such as an Instant Pot, cook at high pressure for 20 minutes for tender ribs or, if you prefer ribs that are falling-off-the-bone tender, cook for up to 30 minutes. When the timer goes off, turn the cooker off. Do not let the pressure cooker automatically switch to the “warm” setting.

Step 3

Use the natural method to release the pressure in the cooker. Unlock and remove the lid. Using tongs, move the ribs, bone-side up, to a rack placed on an aluminum foil-lined sheet pan. Let the cooking liquid in the pressure cooker rest for several minutes to allow the fat to rise to the surface.

Step 4

Meanwhile, preheat the broiler and adjust an oven rack to the top or second position.

Step 5

While the broiler heats, spoon off and discard the fat from the surface of the cooking liquid. Place the stove-top cooker over medium heat or turn the electric cooker to “brown” or "saute" and bring the stock to a vigorous simmer. Cook until the stock is reduced to 1/3 of the original volume, 8 to 10 minutes.

Step 6

Stir in the mustard and brown sugar, and continue simmering for 6 to 8 minutes more, or until the sauce resembles a thick syrup. Remove from the heat.

Step 7

Baste the bone side of the ribs with some of the mustard sauce. Slide the ribs under the broiler until the sauce is bubbling, about 4 minutes. Remove the sheet pan from the oven, turn the ribs over, and baste with the remaining glaze. Return to the broiler until warmed through, about 6 minutes. Cut the ribs into 1- or 2-rib sections and serve.