Stir the lemon juice into the milk; let curdle for 10 minutes.
Cut the broccoli into small pieces and, working in two batches, put in a food processor and pulse until finely chopped. Boil the broccoli just until it is no longer raw tasting, 1 to 2 minutes. Drain in a fine-mesh sieve; squeeze out the excess water. Put in a large bowl, add the milk mixture (homemade buttermilk), mayonnaise and parsley and toss to coat.
Toast the couscous in a dry skillet until it turns deep golden brown, about 2 minutes. Add to the broccoli mixture and season with salt. Stir to combine.