Using a food processor, spice grinder, or mortar and pestle, coarsely grind together the coriander, fennel, and cumin seeds until only a few whole spices remain. Transfer ground spices to a medium bowl and toss with the red pepper flakes, 1½ teaspoons salt, and a few grinds of pepper. Zest 1 lemon to make 3 teaspoons, add into the spice mix, and stir to combine. Then, cut both lemons into wedges for serving.
Add the beef to the spice mixture, tossing to coat and pressing to adhere, as needed. Thread the beef, red onion, and bell peppers onto 12-inch wooden or metal skewers, then add the lemon wedges at the end of the skewers.
Drizzle the skewers with olive oil and season with salt and pepper.
Prepare a 2-zone grill (one side with hot coals and one side without) or heat a grill pan to medium-high. Oil the grates with olive oil. Grill the skewers, turning every 2 minutes, until beef is medium rare, about 8 minutes (if the meat flares up, move it to the indirect heat). Squeeze the grilled lemon over the kebabs and serve.