In a bowl stir together pecans, 1/2 tsp salt, and 1/8 tsp cayenne. In a dry small heavy skillet or saucepan cook 3 tbsp sugar over moderate heat, stirring with a fork, until melted and cook, without stirring, swirling skillet or pan, until a golden caramel. Add pecan mixture and stir to coat nuts with caramel. Spoon praline onto a sheet of foil and cool. Transfer praline to a cutting board and chop fine. Praline can be made 3 days ahead and kept in an airtight container.
Remove tough muscle from side of each scallop if necessary and halve any large scallops. On a sheet of wax paper combine flour, salt, cumin, and cayenne and dip flat sides of each scallop into mixture to coat, knocking off excess. In a skillet heat butter and olive oil over moderately high heat until foam subsides and sauté scallops, flat sides down, until golden and just cooked through, about 2 minutes on each flat side. Remove skillet from heat and cool scallops slightly.
In a large bowl whisk together extra-virgin olive oil, mustard, and salt and pepper to taste until emulsified. Peel and pit avocado and cut into 1/2-inch-thick wedges. Cut wedges in half crosswise and add to dressing. Add scallops with any liquid remaining in skillet, tatsoi or spinach, and praline and gently toss to coat.