Mix the salt, sugar, chili flakes and rosemary in a bowl.
Grind the peppercorns, fennel and coriander seeds in a small spice grinder. Stir into the bowl.
Peel the lemon thinly with a vegetable peeler, avoiding the white pith as much as possible. Slice the peel into thin matchsticks, then slice crosswise to make tiny squares.
Stir the lemon into the salt mixture. Spread out on a rimmed baking sheet in one layer. Let the mixture dry for a day or two before packing into a small jar.
Notes
Seasoned salt will keep 3-4 months. Be sure to store in a sealed jar.