Joi's Cooking Pressure Cooker Pot Roast

FOR THE POT ROAST:
4 pounds beef chuck roast, cut in half
Kosher salt and black pepper
2 tablespoons vegetable oil
7 ounces frozen pearl onions (or 1 yellow or red onion, peeled and cut into wedges)
8 garlic cloves, smashed and peeled
1 ½ cups dry red wine
1 tablespoon red wine vinegar
5 fresh woody herb sprigs, such as a mix of thyme, sage and rosemary (or 1 teaspoon dried thyme, rosemary or sage)
1 to 1½ pounds root vegetables, such as a mix of peeled carrots and parsnips, cut into 2-inch chunks
2 tablespoons unsalted butter, at room temperature (optional)
2 tablespoons flour (optional)
FOR THE PICKLED ONIONS (OPTIONAL):
1 cup red wine vinegar
2 tablespoons granulated sugar
7 ounces frozen pearl onions (or 1 yellow or red onion, peeled and thinly sliced)
Kosher salt