Place 2 tablespoons unsalted butter on the counter to soften at room temperature. Thinly slice 2 medium yellow onions and mince 3 garlic cloves. Pick the leaves from fresh thyme sprigs until you have 1 teaspoon.

Heat 2 tablespoons olive oil with the Sauté setting in a 6-quart or larger Instant Pot or electric pressure cooker until shimmering. Season 1 (3-pound) tied boneless beef chuck roast with 1 ½ teaspoons kosher salt and 1 teaspoon black pepper. Add the beef to the pot and cook until deep golden brown on all sides, 7 to 9 minutes total. Transfer to a plate.

Add the onions and sauté until golden brown, 8 to 10 minutes, adding the thyme and garlic during the last minute of cooking. Add 2 cups of the beef broth, scraping any browned bits up from the bottom of the pot. Return the chuck roast to the pot.

Lock the lid on and make sure the valve is set to seal. Set to cook under HIGH pressure for 60 minutes. When the cook time is up, let the pressure naturally release for 5 minutes. Quick release any remaining pressure.

Transfer the roast to a cutting board and let rest for 10 minutes. Meanwhile, add the remaining 2 cups beef broth to the pot and simmer on the Sauté function to let the sauce reduce for 10 minutes. While the sauce is reducing, mash the butter with 2 tablespoons all-purpose flour until a paste forms. Heat the broiler.

When the sauce is reduced, whisk in the butter paste and simmer until slightly thickened, 2 to 3 minutes. Meanwhile, cut 2 to 3 baguettes into 6 (6-inch) pieces, then split each piece lengthwise to form sandwich rolls.

Place the split rolls on a baking sheet. Place a few slices of Swiss cheese, if using, on the top half of each roll. Broil until lightly toasted and the cheese is melted, about 2 minutes. Thinly slice the roast across the grain. Divide the beef between the baguettes and drizzle with some of the onion broth. Serve the onion broth on the side for dipping.