Rinse the brisket and dry it completely with paper towels. Trim off the excess fat, leaving about 1/4-inch. Season it on both sides with salt and the pepper to taste.
Press the “Sauté” button on your Instant Pot, set to highest heat setting, and add the oil to the pot. (If using a stovetop pressure cooker, heat oil over medium high heat.)
Place the brisket in the pot with the side with the most fat facing down, and let it sear for about 5 minutes until it is a dark brown. Use a pair of tongs to turn it over and sear it for 5 additional minutes. Transfer brisket to a plate.
Lower the Instant Pot to a medium "Sauté" setting. (If using a stovetop pressure cooker, lower the heat to medium.) Add another teaspoon of oil if necessary, then add the sliced onions, garlic, and 1 teaspoon of salt. Cook 8-10 minutes, stirring frequently, until the onions soften and beginning to caramelize.
Deglaze the pot with the red wine, scraping the brown bits from the bottom. Add the broth, orange juice and zest, cranberries, brown sugar, and herbs.
Carefully lower the brisket back into the pot, leaving the side with the most fat facing up. Spoon a bit of the cooking liquid over the meat, then place the lid on the pot, being sure the pressure regulator is sealed.
Press the “Manual” button, then set the time to 50 minutes at high pressure. (For stovetop pressure cookers, cook for 45 minutes at high pressure.)
When the cooking time is over, let the pot depressurize naturally over 20 minutes, or perform a quick pressure release by moving the vent from “Sealing” to “Venting.” (For stovetop pressure cookers, perform a quick pressure release.)
Transfer the brisket to a plate or carving board. Strain the cooking liquid into a fat separator, or into a bowl that you can skim the fat from the top in as it cools. Place the onions, garlic, and cranberries in a bowl, then add only the cooking liquid. With an immersion blender, blend into a thin, slightly chunky gravy. Set aside.
Discard all but 1/4 cup of the fat and place remaining fat back in the Instant Pot. Set it to medium "Sauté. Dissolve the cornstarch in the water, then whisk it into the fat. Continue to whisk until the gravy begins to thicken, then slowly add back in the thinned gravy. Alternate stirring and adding the gravy, maintaining the consistency you want, for 3-5 minutes. Season to taste with salt and pepper, adding more brown sugar as needed also if it's too tart.
Slice the brisket against the grain into thin slices. Pour the gravy over the brisket and serve hot with Merlot Pearl Onions.