To make the dressing, combine the olive oil, balsamic vinegar and salt in a screw-top jar. Close the jar and shake well to combine.
Place the lemon juice in a medium-sized bowl. Cut off and reserve a few of the freshest looking fennel fronds for garnish. Trim the fennel bulbs by removing any green stalks. If the base of the fennel bulb is a little brown, cut a thin slice off. Use a vegetable peeler to remove any brown parts from the sides of the fennel. Cut the fennel bulb in half, then thinly slice using a mandolin or knife and place into the bowl with lemon juice.
Thinly slice the pears and also place into the bowl with lemon juice. Toss to coat the fennel and pears in lemon juice. This prevents oxidation or browning of the fennel and pears.
Add the mixed salad leaves and walnuts to the bowl with fennel and pears, then pour over the dressing. Toss to combine, then transfer to a serving platter or bowl.
Serve the Insalata mista, finocchio, pera e noci (salad of mixed lettuce, fennel, pear and walnuts) with shaved Parmigiano Reggiano and reserved fennel fronds on top – buon appetito.