MAKE THE VINAIGRETTE:
• Place the olive oil, minced shallot, mustard, lemon juice, smoked paprika, salt and pepper into a glass jar fitted with a lid. Screw the lid on and shake the ingredients until the vinaigrette is emulsified. You can also use a bowl and a whisk.
MAKE THE SALAD:
• Use a large, sharp knife to slice the peel off the grapefruits (be sure to remove all the white pith), then cut between the membranes to release the grapefruit segments. This can be done ahead of time.
• Just before serving, cut the avocados in half and remove the pits, and carefully peel off the skin. Cut each half into 4 thick slices. Toss the avocado slices in the vinaigrette to prevent them from turning brown.
• Place the arugula onto a plate or plates. Layer the avocado and grapefruit pieces. Top with the thinly sliced red onion.