Heat a skillet with oil over medium-high heat.
While skillet is heating, season meat on both sides with salt and black pepper.
Add beef to heated skillet and sear on both sides until deep golden brown and lightly crusted, about 5 minutes per side.
Transfer meat to the bowl of a slow cooker (leave the skillet over heat).
Reduce heat in the skillet to medium and add butter. Stir until butter is melted.
Add onions to butter and cook, stirring constantly, until onions are soft and golden brown, about 4 minutes.
Add flour and stir until no dry spots of flour remain, about 1 minute.
Add 3 tablespoon of liquid from a jar of pepperoncini to the pan, scraping up any brown bits from the bottom of the pan (these will add extra flavor to the roast).
Scrape onions into the bowl of the slow cooker.
Add sliced pepperoncini and the ingredients for the Ranch Herb Blend.
Cover and cook on low for 10 to 12 hours, until roast can be easily shredded with a fork.
Transfer to a cutting board and shred.
Place shredded meat in a serving bowl and drizzle cooking liquids from the slow cooker over top.