Make the marinade and marinate. Combine the orange zest and orange juice with the lime juice in a medium-sized bowl. Add the halibut and gently mix to be sure all of it is covered with the citrus juices. Cover the bowl tightly with plastic wrap, place it in the refrigerator, and let it marinate until the halibut is opaque in color, about 8 hours, and ideally overnight. (You are essentially "cooking" the fish.)
Add remaining ceviche ingredients. Add the tomatoes, cilantro, scallions, jalapeños, garlic, olive oil, cumin and cayenne. Then season to taste with salt and pepper.
Marinate again (quickly). Cover the ceviche with plastic wrap again, and let it marinate in the refrigerator for about 15 minutes.
Make taco shells. To make the taco shells, coat the bottom of a large sauté pan with olive oil and place it over medium-high heat. Add as many of the tortillas as you can easily fit. Sauté each side for a minute or so, until they are crisp and golden. Use kitchen tongs or a fork to fold each one in half. Repeat until all of the tortillas are cooked. As you remove them from the pan, place them on a paper towel-lined plate to soak up any excess oil.
Assemble. When you're ready to serve, add a few baby greens, about 1½ tablespoons or so of the ceviche, and an avocado slice (or two).