Pat the roast dry with paper towels, then season generously with salt and pepper at least 15 minutes before searing.
Smash the garlic cloves, quarter the onion, and measure out the pepperoncini and juice.
Heat olive oil in a large skillet over medium-high heat. Sear the roast on all sides for 2 to 3 minutes per side until a dark brown crust forms.
Place the seared roast in the slow cooker. Add garlic, onion, bay leaves, pepperoncini peppers, pepperoncini juice, and beef broth around the meat.
Cover and cook on low for 8 hours, until the meat shreds easily with a fork. At the 4-hour mark, flip the roast for even cooking.
Remove the roast to a cutting board and shred into bite-sized pieces with two forks, discarding large fat chunks.
Strain the cooking liquid through a fine-mesh sieve into a bowl, pressing the onion and garlic to extract flavor, then discard solids.
Return shredded meat and strained sauce to the slow cooker. Stir in cold butter until melted and coats everything. Taste and adjust salt.