Place sliced shallots in a colander in the sink. Slowly pour boiling water over shallots. Transfer to a small bowl and cover with the vinegar and let stand 20 to 30 minutes. Drain off all the vinegar and reserve for dressing.

Line a toaster oven pan with parchment paper. Spread pepitas evenly over parchment. Toast on high for three to four minutes, until puffy and making crackling sounds.

Combine two tablespoons plus one teaspoon leftover vinegar with olive oil, mustard, salt, and pepper. In a salad bowl, toss the lettuce with two tablespoons of the dressing. Arrange lettuce leaves on four salad plates.

To make cucumber ribbons, lay cucumber on its side, perpendicular to you on a cutting board. Shave four ribbons, moving the peeler away from you, discard the first ribbon with all the skin. Turn cucumber over and repeat making four ribbons on the other side. Lay the 24 ribbons out in a single layer on paper towels and blot the tops with paper towels while the rest of the salad is prepared.

Add cucumber, avocado, and tomato halves to the salad bowl and toss with remaining dressing. Arrange mixture on top of the lettuce. Arrange about two tablespoons pickled shallots on each salad and sprinkle salads with one tablespoon each of pepitas before serving.