To make the dressing, combine the lime zest and juice, salt, cumin, paprika, and mustard in a bowl or jar and whisk or shake to combine and dissolve the salt.

Add the olive oil to taste and whisk or shake until emulsified.

Place the torn lettuce leaves in serving bowl.

Using a sharp knife, slice both ends off of the grapefruit, then stand the grapefruit on a cutting board and slice the rind off of the flesh. Hold the grapefruit over the serving bowl containing the lettuce leaves and slice the sections of the grapefruit into the bowl by slicing along side the membrane between the sections (so that the flesh drops into the serving bowl, leaving the membrane behind.)

Stir the diced avocado into the dressing, then spoon dressing (including the bits of avocado) onto the salad (you may have extra dressing, but make sure you scoop all of the avocado bits out before refrigerating any extra dressing, which will last this way in the refrigerator, covered, for 2 or 3 days).

Toss the salad very gently to coat evenly with the dressing and serve immediately.