Use a sharp paring knife to slice off the top and bottom of each lemon and then set them upright on the cutting board. Cut down the length of each lemon all around, in essence peeling it, removing both the layer of skin and the white pith underneath. Cut the lemon flesh into sections, slicing between the membranes to remove the wedges of lemon, leaving any fibrous pieces and pits behind. Squeeze any juice from the empty membranes into a bowl with the sections and discard the membranes.
Make the slaw:
In a large bowl, whisk together the mayonnaise, skim milk, poppy seeds, and red pepper flakes. Stir in the celery root, apples, lemon sections, and parsley. Season with salt. Cover with plastic wrap and refrigerate.
Prepare the salmon:
Use half of the olive oil to grease the bottom of a roasting pan. Arrange the salmon fillets, skin side down, in the pan. Dot them with the butter and drizzle with the rest of the oil. Season with salt and white pepper.
Cook the salmon:
Roast the salmon in the center of the oven, undisturbed, until medium-rare, 8 minutes. (For well-done fish, leave it in the oven for an additional 5 minutes.) Remove the pan from the oven and transfer the salmon to a serving platter. Set the salmon aside to rest for 5 to 10 minutes. When ready to serve, arrange the slaw on top of the salmon. Serve immediately.