The intense flavor of the pesto is mellowed a bit by adding it to the vegetables in the oven for a few minutes.
Heat the oven to 375.
Slice 6 carrots in half lengthwise, then cut into 2″ pieces and arrange in a single layer on a well-oiled cookie sheet. Sprinkle with salt and pepper and roast for 10 minutes.
Snap the bottoms off 1 bunch of asparagus. Cut the spears in 2″ pieces. Trim 1 medium spring onion and then cut the entire thing, white and green parts. Arrange the asparagus in between the carrots, then sprinkle the onions over everything. Continue to roast another 15-20 minutes, until the asparagus begins to brown.
Meanwhile, make the cilantro pesto: Chop 1 bunch of cilantro, leaves and stems, and place in a small food processor or mini-chopper. Mince 1 pencil-sized stem of green garlic, white and green parts, and add to the chopper. Add 1/2 t. salt, then puree the cilantro and garlic while drizzling in 2 T. olive oil.
Add 3 T. of the pesto to the roasting vegetables and stir to combine. Return to the oven for just 3-5 minutes — you want the pesto to sizzle but not burn.
Place 2 C. dried couscous in a large bowl. Boil water, then pour it over the grain until it is just barely submerged. Cover the bowl and let sit for 5 minutes, then uncover it. Fluff with a fork and let sit another 5 minutes.
Toss the vegetables and couscous together. Taste and add salt, more pesto, and lemon juice to taste. Add 1/2 C. crumbled feta cheese and toss again.