Toss the sliced beef with the coriander, grated garlic, 1 tsp salt, tamarind paste and brown sugar. Marinate 15 minutes at room temperature. Just before frying, add the cornstarch and toss until every slice is lightly coated.
Blitz the red chilies, bird's eye chilies, the 3 chopped shallots, 4 garlic cloves and the tomato in a food processor into a coarse paste. Do not puree it smooth, the texture should be rough.
Heat 2 tbsp oil in a wide pan over medium heat. Cook the paste, stirring, 8 to 10 minutes until the raw smell is gone and the oil separates at the edges. Stir in the sugar, 1/2 tsp salt and the sliced shallot wedges, cook 2 minutes more, then turn off the heat and stir in the lime juice.
Heat 1/3 cup oil in a large skillet or wok over medium-high heat until a slice sizzles right away. Fry the beef in 4 batches, spreading the slices in a single layer, 1 to 2 minutes per side until deep brown with crisp edges. Drain on a rack or paper towels.
Pile the crisp beef on a platter and spoon the warm sambal over the top, or toss everything together in the pan for 30 seconds. Serve right away with rice and sliced cucumber.