Toss 1 large English hothouse cucumber, very thinly sliced into rounds, 1 Tbsp. fresh lime juice, and ½ tsp. Diamond Crystal or ¼ tsp. Morton kosher salt in a medium bowl to combine, massaging with your hands until cucumber begins to soften and become more pliable. Set aside.

Scoop flesh from 1 large avocado, halved, pit removed, into a blender; add 4 scallions, coarsely chopped, 1 garlic clove, smashed, 1 jalapeño or serrano chile, stem removed, ½ tsp. ground cumin, ¼ cup vegetable oil, 1 tsp. Diamond Crystal or ½ tsp. Morton kosher salt, remaining 3 Tbsp. fresh lime juice, and 3 Tbsp. room temperature water and blend until smooth. Add 2 cups (lightly packed) cilantro leaves with tender stems and blend just until mostly smooth with some flecks of cilantro remaining. Taste sauce and add more lime juice and/or season with more salt if needed.

Season four 4–6-oz. skin-on salmon fillets generously with salt. Pour remaining 1 Tbsp. vegetable oil into a cold large nonstick skillet, arrange salmon, skin side down, in pan, and place over medium heat. Cook, undisturbed, until skin is deep brown, 8–10 minutes. Turn and cook on other side until surface is opaque, about 1 minute. Remove from heat.

Divide sauce among plates; place salmon, skin side up, on top. Scoop reserved cucumbers over with a slotted spoon.