Heat the oven to 400 degrees with a rack set in the lower third. On a sheet pan, combine the broccolini, tomatoes, onion and lemon slices with the olive oil and toss. Add cumin and red-pepper flakes, season with salt and pepper, and toss again until evenly coated. Nestle the feta slices into the vegetables. (It’s OK if they break apart a little.) Roast 15 to 20 minutes, stirring halfway through but leaving the feta in place, until the broccolini is charred at the tips, the stems are easily pierced with a fork and the tomato skins start to blister and break down. Serve over orzo or farro. Drizzle with olive oil and serve with the remaining lemon half for squeezing. Top with fresh herbs, if using.