Spread lemon and orange zest on a baking sheet; dry in oven for about 10 minutes. Place fennel seeds in a pan; roast in oven without oil until fragrant. Let cool.
Using a mortar and pestle, combine fennel, sea salt, sugar and hot pepper flakes; grind until fine. Add thyme and dried citrus to mortar and pestle; grind until fine. Mix with paprika and garlic.