In a small bowl, combine 2 tsp kosher salt, 2 tsp black pepper, 2 tsp garlic powder, 2 tsp dried oregano, 0.5 tsp crushed red pepper flakes, 0.5 tsp lightly crushed fennel seeds, and 1 tsp onion powder. Rub this mixture all over the beef.
Heat 1 tbsp olive oil in a heavy oven-safe roasting pan over medium-high heat. Sear the beef on all sides until well browned, 6–8 minutes total.
Transfer the pan to the oven and roast until the internal temperature reaches 125–130°F for medium-rare, 1 hour 30 minutes to 2 hours depending on thickness. The beef should feel springy, not firm.
Remove the beef to a plate and rest 20 minutes, then wrap and chill until very firm for thin slicing, 1–2 hours (or freeze 30 minutes to speed firming).
While the beef chills, place the roasting pan over medium heat. Add 64 ounces low-sodium beef stock and scrape up browned bits. Stir in 4 sliced garlic cloves, 2 bay leaves, 1 tsp dried oregano (reserve from total), 0.25 tsp crushed red pepper flakes (reserve from total), and 1.5 tbsp Worcestershire sauce. Simmer gently, uncovered, 20–30 minutes until flavorful; skim excess fat. Season with 0.5 tsp kosher salt if needed. Strain the jus and keep hot over low heat.
Make the sweet peppers: Heat the remaining 2 tbsp olive oil in a large skillet over medium heat. Add the sliced green bell peppers, the remaining 1 sliced garlic clove, and 0.5 tsp kosher salt. Cook, stirring occasionally, until tender with a few golden edges, 10–12 minutes. Keep warm.
Thinly slice the chilled beef across the grain as thin as possible (1–2 mm) using a sharp knife or slicer.
Bring the jus to a bare simmer (do not boil). Working in batches, swish handfuls of sliced beef in the hot jus just until warmed through and no longer cool in the center, 1–2 minutes per batch. Transfer beef to a warm bowl and ladle a little jus over to keep it moist.
Warm the Italian rolls in the oven for 3–5 minutes until the crust is just crisp. For each sandwich, pile a generous portion of jus-soaked beef onto a roll.
Top with warm sweet peppers or spoon on oil-packed giardiniera (or both). For a "wet" or dipped sandwich, briefly dip the filled sandwich or just the cut side of the roll into the hot jus. Serve immediately with extra jus on the side.