Add beer, pomegranate molasses, garlic and red pepper flake to a large bowl, stirring to combine. Submerge tri-tip into the marinade. Cover and refrigerate overnight. Flip tri-tip in the marinade halfway through the waiting period (in the morning).

If you are using a charcoal grill, be sure to spread your briquettes into a slope. Half of the grill will be used for direct cooking (to get a nice crust on the outside) and the other half will be used for indirect cooking (to fully cook the meat without burning it).

About 30 minutes before you plan on putting the tri-tip on the grill, take the meat out of the refrigerator. Remove the tri-tip from the marinade and set aside to bring to room temperature. Pour the marinade into a small, heavy bottom pot and bring to a boil over medium-high heat. Reduce the heat to low and simmer until the mixture has reduce by half (you can simply simmer the sauce until you are ready to use it - see below).

Season the steak generously with kosher salt and freshly ground black pepper. Once the tri-tip has come to room temperature, place it on the hottest part of the grill. Cook for 5 minutes on each side, then move to the cooler part of the grill. Baste with reduced marinade. Cover and cook until thermometer inserted into the thickest part of the beef reaches 135°F (for medium-rare), turning occasionally and basting with marinade (15 - 30 minutes depending on the thickness of your tri-tip).

Transfer the meat to a cutting board and let stand for 5 minutes. Slice across the grain and serve immediately.