Step 1

Remove tough outer layers from lemongrass. Trim off top third of stalk; discard. Bruise remaining stalk by whacking a few times against a cutting board. Transfer lemongrass to a small saucepan and add shallot, garlic, ginger, oil, and salt and bring to a gentle simmer over medium heat. Cook, reducing heat as needed to keep at a low boil, until shallot is soft and just beginning to turn golden around edges, 15–20 minutes. Remove from heat and add chiles and red pepper flakes, pushing down with a spoon and stirring occasionally until chiles soften slightly. Purée in a blender until chiles are broken down to small flecks (be careful not to overprocess). Transfer to a small bowl; stir in peanuts and fish sauce.

Step 2

Do Ahead: Peanut-chile oil can be made 1 week ahead. Cover and chill. Bring to room temperature before using.

Step 3

To make them easier to slice, freeze ribs just until very firm around edges, 20–30 minutes.

Step 4

Meanwhile, remove tough outer layers from lemongrass. Finely grate bottom third of stalk; discard remainder or freeze for another use. Combine lemongrass, garlic, ginger, coconut milk, fish sauce, brown sugar, lime juice, and turmeric in a large resealable plastic bag. Slice short ribs lengthwise against the grain into ¼"-thick strips and add to bag. Seal bag, pressing out air, and chill at least 2 hours and up to 2 days.

Step 5

Prepare a grill for medium-high heat; oil grates. Remove beef from marinade, letting excess drip off. Thread onto 6–8 skewers, folding back and forth in accordion style; season lightly with salt. Grill, turning once, until browned and just cooked through, about 5 minutes. Serve beef with peanut-chile oil.