Step 1
Remove zest from lemon in wide lengthwise strips with a vegetable peeler, avoiding as much white pith as possible. Cut lemon in half and squeeze juice through a fine-mesh sieve or your hand into a small bowl; discard seeds. Set juice aside.
Step 2
Heat lemon zest, garlic, red pepper flakes, and oil in a medium saucepan over medium-low, stirring to break garlic apart and reducing heat if oil around aromatics starts to sizzle too much, until fragrant, 10–12 minutes. (The garlic should gently cook but not take on much color.)
Step 3
Remove saucepan from heat and add beans, reserved lemon juice, and salt; season with black pepper. Toss to combine; let sit at least 15 minutes.
Step 4
Meanwhile, toast pistachios in a dry small skillet over medium heat, tossing occasionally, until golden brown, 5–8 minutes. Transfer to a cutting board and coarsely chop.
Step 5
Combine radicchio, brussels sprouts, and ¼ cup dill in a large bowl. Crumble in feta in large pieces and toss to combine. Using a slotted spoon, add beans, garlic, and lemon zest. Drizzle with infused oil and toss to coat.
Step 6
Mound salad on a platter; top with pistachios and remaining dill.