How to Make It
Coarsely chop leaves from 1 lb. rainbow chard to equal 8 cups; thinly slice stems to equal 1 cup. (Reserve remaining stems for another use.)
Heat 1 Tbsp. olive oil in a large skillet over medium-high. Add chard stems and 1 cup thinly sliced fennel bulb; cook, stirring often, until starting to soften, about 3 minutes. Add chard leaves and 1/4 cup water. Cover and cook until just starting to wilt, about 1 minutes and 30 seconds. Drain. Top servings evenly with 1/4 cup chopped pistachios, 2 Tbsp. fennel fronds, 1 Tbsp. apple cider vinegar, 1/4 tsp. black pepper, and 1/8 tsp. kosher salt.