Remove scallops from refrigerator, pat dry, and allow to come to room temp. Season with salt and pepper on both sides.

Trim top and bottom from lemon and place a flat side onto cutting board. Working along the curve, slice skin-pith layer from flesh in strips, turning as you go. Squeeze any juice in the strips into a large bowl then discard. Cut lemon into supremes into the bowl, then squeeze the juice from the membrane.

Cut fennel in half lengthwise. Use a man-doline placed over the bowl to shave fennel, reserving the fronds. Add a drizzle of oil and the mustard, and season with salt and pepper. Gently toss the mixture to combine.

Heat a cast-iron skillet over high heat until near smoking. Add a glug of olive oil and quickly swirl, then place scallops into pan equidistant from one another. They should immediately vigorously sizzle. After a minute or two - do not disturb them - add 1 tablespoon butter, dragging it in between each scallop. Sear for 3 more minutes, tilting pan once or twice to keep the hot fat flowing around them. Turn scallops over and baste their caramelized surfaces with spoonsful of the hot fat. Repeat basting as you sear for one minute longer, then remove scallops from heat, transfer to a plate, and pour pan juices over.

Cut peaches into slices. On two plates, divide fennel mixture and layer with peach slices and reserved fennel fronds, and nestle scallops on top, pouring the pan juices over all. Add a final sprinkle of sea salt and freshly cracked pepper and eat at once.