For the croutons: Preheat the oven to 375 degrees F.
Peel the sweet potato and cut into small 1/2-inch cubes. Place on a baking sheet and toss with the oil and a sprinkle of salt and pepper. Bake until they are crisp on the outside, about 20 minutes.
For the mushrooms: In a large sauté pan, add the oil, then tip in the tamari, vinegar, thyme, mushrooms and garlic. Sauté until the juices just evaporate and the flavors infuse, 8 to 9 minutes.
For the tabouli: In the meantime, grate the cauliflower into a mixing bowl and add the parsley, lemon juice and a pinch of salt. Mix well and set aside until ready to assemble the cups.
To assemble the salad cups, take a lettuce leaf, fill with a spoonful of the cauliflower tabouli, then a spoonful of the warm mushrooms and top with a scattering of the sweet potato croutons. Repeat with the remaining lettuce, tabouli, mushrooms and croutons.