Combine the lime zest and juice, tequila, sugar and salt in a medium glass or other non-reactive (non-aluminum) bowl. Toss in the halibut. Cover, refrigerate and marinate 3 to 4 hours, or until the fish has turned opaque and appears cooked from the acid in the marinade.
Drain and discard all but 1 Tbsp. of the liquid in the bowl. Now toss in the onion, cilantro, fresh peppers, oil, tomatoes and avocado. Divide the ceviche mixture among six decorative (about 6 oz) glasses, garnish with lime slices and serve.
Options: If desired, before spooning the ceviche into the glasses, you could coat the rim of each glass with sea salt. To do so, dip the rim of each glass in lime juice, and then in coarse sea salt.