Fill a Collins glass with ice and add 1 oz mezcal, ½ oz Grand Marnier, 2 oz orange juice, 2 oz tangerine juice, and ¼ oz kumquat syrup. Add ¼ oz grenadine and let it settle to the bottom of the glass. Garnish with a kumquat or orange wheel. .
Slice kumquats into ¼-inch-thick (5-mm) rounds. Using a skewer, poke out seeds.
In a small saucepan, bring water and sugar to a boil over medium-high heat, whisking to dissolve sugar. Add kumquat slices. Reduce heat to maintain gentle simmer. Cook, stirring occasionally, until kumquats are tender, about 12 minutes. Remove from heat and cool completely.
Transfer kumquats and their syrup to a glass jar with lid. Refrigerate up to 1 month.