If using bamboo skewers soak in water for one hour before grilling. Thread the lamb cubes on skewers alternating with pieces of red onion and bell pepper. Discard lamb marinade. Set aside at room temperature while preparing the grill.
Preheat grill on high for 10 to 15 minutes. Clean and generously grease the grill grates using folded paper towels soaked in vegetable oil. Use long grilling tongs for oiling the grates to prevent burns.
Cut the lemons and tomatoes in half. Lightly brush the cut sides with vegetable oil. Grill, cut side down, until well charred. Remove to a clean pan and set aside.
If needed, oil the grill grates again. Grill the lamb skewers turning occasionally until charred on all sides. Remove from the grill. Quickly heat the pita bread on the grill until warmed through.
To serve, brush or drizzle some of the remaining marinade sauce over the Souvlaki.
To make a pita sandwich: Hold a warm pita in your left hand, and the skewer in your right. Pull the contents from the skewer into the pita bread. Top with french fries, if using, tzatziki sauce, a squeeze of grilled lemon juice and a few tomatoes. Drizzle with more reserved marinade, if desired. Serve!