With a zester or Microplane grater, scrape some grapefruit zest from the skin of the fruit for garnish, and reserve. Segment the grapefruit: trim off the bottom and top of the fruit with a knife so that you have a flat surface upon which to rest it as you peel it. Peel the fruit by placing the tip of a sharp knife just inside the border where the pith meets the pulp, and slicing down with firm, clean strokes following the curvature of the fruit. Repeat until the entire fruit has been peeled. Then, over a bowl or wide board to catch all of the juice, gently cut the segments of pulp with a sharp knife by slicing toward the core as close as possible to the membranes that separate the segments. Once you’ve extracted all the citrus segments, squeeze the membranes to release any remaining juice and then discard the membranes. Gently strain the segments, reserving segments and juice in separate bowls. Add ¼ teaspoon salt, the vinegar, 1 tablespoon of water, and the mustard to the bowl with the grapefruit juice and whisk to combine. Pour in the olive oil, whisking to emulsify.
Pour the canola oil into a large skillet over high heat, and when it smokes, add half of the asparagus and ¼ teaspoon salt, and cover. Cook, partly covered, until the asparagus is blackened on one side, 3 to 4 minutes. Turn the asparagus in the pan, cover, and cook until the asparagus is thoroughly blackened, 3 minutes more; transfer to a serving platter. Repeat with the remaining asparagus, adding another teaspoon of oil to the pan (if it’s become too dry) and seasoning with salt.
When all the asparagus is on the platter, scatter the grapefruit segments evenly over the asparagus. If the dressing has broken, whisk to re-emulsify, pour it over the asparagus, and grind some black pepper over the top. Garnish the platter with the reserved zest, and serve.