Toss the sugar and zest together on a rimmed plate and wipe the rim of the glass with the squeezed carcass of your Seville oranges. Lightly press the inverted glass into the sugar so it adheres to the rim.
Next, and quickly, fill an 8-ounce tumbler glass with ice and pour the juice, tequila, and agave into a shaker. Fill it with lots of ice, cover, and shake 7 - 8 times only. Strain into the glass right away. You can garnish with a slice of Seville if you feel so inclined or just get right to sipping, imaging that winter is just a thing of the past.
* The grated rind, squeezed juice, and even whole fruit itself freeze beautifully. Juice should be strained, rind frozen in a single layer on a parchment or plastic wrap-lined sheet pan (or similar), and whole oranges double wrapped tightly in plastic wrap and then in a layer of heavy duty aluminum foil.