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Place fennel, spices, sugar, vinegar, and water into a small saucepot and bring to a boil over medium-high heat. Meanwhile, as your saucepot is heating, zest the navel orange. It should yield about 1 teaspoon, but if you get less, that is fine. Set zest aside. Peel the orange as if you were supreming or segmenting it, but instead of segmenting, cut the orange into 4 pieces and remove the middle pithy part, seeds, and hard rind (if any). The membrane between the orange segments is fine. Dice what you have, which should yield about 1 cup. Add to the saucepot, which should have come up to a rapid simmer/boil about now. If the pot started boiling while you were cutting up the orange, that is fine. Once the oranges are in, bring to a boil for about 5 minutes, skimming any foam that appears, then turn down to medium. Simmer until liquid is reduced to the consistency of maple syrup (nearly all of the liquid will be gone by then) and the mustard seeds have plumped up and softened, about 20-25 minutes. Set aside and cool, then stir in reserved orange zest. Note: At this point, there will still be pieces of fresh orange in the mostarda. If you want a more cooked-down, marmalade-ish consistency, bring the orange to a boil with the fennel, and simmer on medium-low heat for 30 minute
