Season the porterhouse steaks generously with coarse sea salt and place them in the refrigerator until ready to grill.
Drizzle the cut side of the garlic bulbs with canola oil and season with sea salt and black pepper. Wrap tightly in aluminum foil.
Preheat the grill for two-zone cooking at high heat, about 400°F.
Place the wrapped garlic bulbs on the cooler side of the grill, cut side down, and roast for 30–40 minutes until soft and golden.
Remove the roasted garlic from the foil and press the softened garlic into a bowl. Add rosemary, thyme, sage, chili flakes, red wine vinegar, spicy mustard, olive oil, and kosher salt. Mix thoroughly to create the board sauce.
Place the steaks over direct heat and grill for 3–4 minutes per side, or until the internal temperature reaches 120°F for medium-rare.
Melt the butter and baste the steaks during the final minutes of cooking, flipping frequently for even flavor and crust development.
Spread the board sauce onto a cutting board. Place the hot steaks directly on top of the sauce and let rest for 2–3 minutes.
Flip the steaks to coat both sides with the sauce, then slice and serve immediately.