Charring citrus will give a slight smoky flavor to this easy one-pan dinner. INGREDIENTS: 1 lemon 2 bulbs fennel, thinly sliced 2 small red onions, thinly sliced2 1/2 tbsp. olive oil, divided Kosher salt and pepper 1 1/4 lb. skin-on salmon fillet 1 tsp. stone-ground mustard 3 c. baby arugula DIRECTIONS : …