Season all sides of chuck roast with lots of pepper and garlic powder, and a little seasoned salt. In large frying pan, add vegetable oil to coat bottom of pan well. Heat over medium-high heat until very hot, but not smoking. Add beef and sear all sides until browned. Place into an appropriate sized slow cooker/crock pot.
Season top side with half of pesto packet, turn over and season with rest. Sprinkle Ranch mix over top, and place slices of butter on top. Place on lid, and cook on low for 7 - 8 hours or until fall-apart tender.
Pull apart meat with two forks and stir to coat evenly with sauce and juices. Place lid back on and continue cooking for another 30 minutes.
In small bowl, add mayo and jarred pesto, stir until well combined. Chill until needed.
When beef is done, top bottom buns with piles of fresh baby spinach leaves. Spread top buns with a nice amount of pesto-mayo.
With a slotted spoon, pile beef onto bottom buns, top with thinly sliced tomatoes and red onion, place on bun top and dig in!