Combine 3/4 cup hot water and the bouillon cube in a small glass baking dish; stir to dissolve. Whisk in 2 tablespoons olive oil, the soy sauce, Worcestershire sauce, the juice of 1 lemon, the red pepper flakes, 1 teaspoon salt and 1/2 teaspoon pepper. Tear 4 sprigs thyme and 3 sprigs rosemary into pieces; add to the marinade along with the scallions. Let cool.
Using a fork, pierce the meat all over at 2-inch intervals. Add to the marinade, making sure it is completely submerged. (If necessary, weigh down the meat with a plate.) Cover and refrigerate 3 to 8 hours.
Remove the meat from the refrigerator about 1 hour before grilling (leave it in the marinade). Put the mushrooms and bell peppers in a large bowl. Tear the remaining 2 sprigs thyme and 1 sprig rosemary into pieces and add to the bowl along with the garlic, the remaining 2 tablespoons olive oil and the juice of 1/2 lemon. Season with salt and pepper and toss.
Preheat a grill to medium, then prepare for indirect heat: For gas, turn off the burners on one side. For charcoal, push the coals to one side. Remove the meat from the marinade and pat dry, reserving the marinade. Place the meat on the hotter side of the grill (direct heat) and cook, turning often, until charred, 15 to 20 minutes; scatter the herbs from the marinade on the meat as it cooks (it's OK if the herbs fall through the grates; they will flavor the smoke). Move the meat to the cooler side of the grill (indirect heat); cover and cook, turning and basting a few times with the reserved marinade, until a thermometer inserted into the thickest part registers 125 degrees F for medium rare, about 20 more minutes. Transfer to a cutting board and let rest at least 10 minutes.
Meanwhile, add the mushrooms and peppers to the grill over direct heat; cook, turning a few times, brushing with the juice of the remaining 1/2 lemon and seasoning with salt, until charred and tender, 10 to 15 minutes. Arrange the scallions from the marinade on the grill and cook 2 to 3 minutes per side. Slice the steak against the grain and serve with the vegetables.