Prep the tea leaves: Drain the lahpet well. If the leaves are whole, chop them finely. Put them in a small bowl with 1 tablespoon of the garlic or peanut oil and a pinch of salt, then mash and stir with the back of a spoon for about a minute — you want a coarse, spreadable paste rather than loose leaves. Taste: if the leaves are sharply bitter, let them sit in warm water for 10 minutes first, drain, squeeze dry, and then dress with the oil.
Crisp the toppings if you are making them: If your fried garlic and fried split peas are store-bought, skip to the next step. Otherwise, heat the remaining 2 tablespoons oil in a small skillet over medium heat. Fry thin garlic slices for 1 to 2 minutes until pale gold, then lift them out to drain — they keep browning off the heat. In the same oil, fry soaked-and-dried split peas for 2 to 3 minutes until crisp. Keep the flavored oil; it dresses the salad.
Soften the onion: Put the sliced onion or shallot in a bowl of cold water for 5 minutes, then drain and pat dry. This short soak keeps the bite but takes off the raw harshness that would otherwise sit on top of the tea leaves.
Build the base: In a wide serving bowl or on a platter, spread the shredded cabbage in an even layer. Scatter the diced tomato and the drained onion over it. Spoon the dressed tea leaves into the center in a mound — the traditional presentation keeps the piles separate until the last moment.
Add the crunch: Scatter the crushed peanuts, fried split peas, sesame seeds, fried garlic, dried shrimp (if using), and sliced chili over the salad. Nothing here should be ground to a powder; pea- to lentil-sized pieces are what give the salad its texture.
Dress and toss at the table: Drizzle the remaining garlic oil over everything, then add the fish sauce, lime juice, and salt. Toss thoroughly with two spoons, folding the tea paste up through the cabbage so the shreds take on color and the tomato juices join the dressing. Taste and adjust: more lime if it feels flat, more fish sauce for depth, a little more salt if your lahpet was mild. Serve right away with lime wedges.