On sauté, melt butter. Sprinkle roast with salt and pepper. Sear roast 1-2 minutes per side. Press "keep warm/cancel." Add onion, garlic, thyme, and wine to roast in Instant pot. Lock lid; set valve to sealing. Program for meat/stew. When IP beeps at the end of the cycle, choose either quick release or natural release. Shred beef. Combine mayo and horseradish. Spread horseradish sauce on bread; top with shredded beef, lettuce, and giardiniera (if desired).