Remove the small, connective muscle on the side of each scallop if it is still attached. You can use fresh or frozen scallops. If they are frozen, no need to thaw them.
Fill a medium saucepan with water and add lemon slices, crab boil mix, and salt. Bring to a boil, then lower heat to a gentle simmer.
Add scallops and return to a simmer. Cook until just barely cooked through, this can take anywhere from 1-5 minutes depending on your starting with frozen scallops or not. Remove one from the cooking liquid and press gently they should still be a little translucent and have a bit of give. It’s better to undercook them than overcook them. They will cook the rest of the way when they get tossed with the lime juice mixture.
Drain scallops and spread in a single layer on a large plate. Chill in the refrigerator while you make the ceviche marinade.
Combine lime juice, tomatoes, avocado, jalapeños, tangerine juice, and cilantro in a large bowl.
Slice each scallop in half horizontally so you have two thinner rounds. Toss scallops in ceviche mixture and chill for at least 1 hour and up to 3 hours in advance.
Right before serving, stir mixture again and taste to see if it needs any more salt or lime juice. Serve on tostada shells and garnish with radish slices.