How to Make It
Light a grill or heat a large grill pan. Drizzle the scallions with olive oil and season lightly with salt. Grill over high heat until nicely charred and just tender, about 2 minutes per side. Coarsely chop the scallions.
In a large deep skillet, heat the 2 tablespoons of olive oil. Add the jalapeños and cook over moderate heat until starting to brown, about 3 minutes. Stir in the chopped scallions, then add the lemon zest, lemon juice and mustard.
In a large pot of boiling salted water, cook the spaghetti until al dente. Drain the pasta, reserving 3/4 cup of the cooking water. Add the spaghetti and the reserved pasta cooking water to the skillet and toss well. Add the butter, season with salt and pepper and toss again. Transfer the spaghetti to bowls and serve, passing the grated cheese at the table.