Preheat the oven to 300 degrees Fahrenheit.: The warm, low oven creates the gentle environment where collagen in the chuck roast breaks down into silky gelatin, producing tender meat and a glossy pan sauce. You should feel a steady warmth from the oven rather than intense heat, which allows even cooking. Listen for the faint hum of the oven as it settles, and avoid opening the door frequently, because dropping the temperature interrupts the long braise and can lengthen cook time.
Season the roast by sprinkling it with salt and pepper. Place olive oil in a large skillet. Over medium-high heat sear the roast for about 2-3 minutes per side, just to get it slightly brown.: As you sear, watch for the meat surface to transform from dull red to deep brown and release a fragrant, toasty aroma. The sizzling sound should be steady, not frantic, indicating proper oil temperature. Searing creates flavor through caramelization, which later infuses the sauce. A common mistake is using too cool a pan, which leads to a gray surface; ensure the oil shimmers before adding the roast.
Place the roast in the roasting pan. Pour on the liquid from the pepperoncinis jar. Sprinkle on the au jus powder and ranch dressing powder.: When the brine hits the hot pan, it will hiss softly and begin to mingle with the roast juices, producing a bright steam and tangy scent. The powdered mixes dissolve into this liquid to build the base of your gravy, and you should see the liquid pick up color and gloss as it warms. Stirring here is unnecessary, but ensure the powders are scattered evenly so the flavors distribute. Avoid adding too much extra liquid, or the braise will become too diluted and the sauce may lack depth.
Cut up the butter and place small chunks of it over the roast. Place the pepperoncinis and garlic on top. Place the lid on the roasting pan or cover it tightly with tin foil.: The visible dollops of butter will slowly melt and baste the meat, creating a rich, velvety sauce; you will notice a glossy sheen form as the fat emulsifies with the cooking juices. The minced garlic will become soft and aromatic rather than sharp, adding savory complexity. Tenting the pan traps steam, which promotes even tenderness; do not leave gaps that allow too much steam to escape, because that can dry the surface.
Bake for 150 minutes. Check for doneness. If it is not easy to tear apart, continue cooking for another 30 minutes.: During this long, patient bake you will smell layers of savory aromas that evolve from roasted meat to deeper, braised notes. Check the roast near the end of this window for visual cues, such as edges pulling away from the meat and the sauce thickening slightly. The fork should pierce with little resistance and the meat should begin to pull apart at the grain. A common oversight is checking too early, before the connective tissue has broken down; if the meat resists, give it more time rather than increasing oven temperature.
Check for doneness. If it is not easy to tear apart, continue cooking for another 30 minutes: When you test doneness, use tongs and a fork and look for that satisfying, shreddable texture; the roast should yield easily and the center should be warm and flecked with softened fat. If it still resists, the collagen needs more time to gelatinize, so extend the cook in 30 minute increments and recheck. Avoid slicing against the grain too early, because that can make the meat feel tougher and dry out the interior.