Using a mandolin, slice the whole cucumber lengthwise very thinly (about ⅛ inch or thinner). You should get about 20 paper thin slices. Lay the slices out on paper towels to dry out a bit. Chop any leftover pieces into matchsticks for the inside of the sushi (optional).
Cook the cauliflower rice until just tender and drain very well to eliminate excess moisture. Stir in the collagen and toasted sesame oil until well combined. Sprinkle with a little salt if desired.
In a medium bowl, mix the crab meat or smoked salmon with the mayo.
Cut a square of plastic wrap or waxed paper. Lay 5 of the paper thin cucumber slices side by side with slightly overlapping edges. Spread with ¼ of the cauliflower rice mixture.
Along one side where the ends of the cucumber slices are, lay some cucumber matchsticks, ¼ of the avocado slices and ¼ of the crab meat. Using the plastic wrap or waxed paper to help lift it, roll as tightly as possible. Tuck in the ends of the plastic wrap or paper. Repeat with remaining ingredients. You should be able to make 4 sushi rolls.
Chill the rolls one hour to allow the cauliflower mixture to firm up. Cut the rolls with a very sharp knife into 6 pieces each. Sprinkle with toasted sesame seeds.
Serve with wasabi and soy sauce or coconut aminos.