Remove leaves from the cauliflower. Carefully trim stem end, leaving core intact so florets are still attached. Place cauliflower head core side down. Cut three 3/4- to 1-inch-thick slices from the center of the cauliflower head. (Store the remainder of the head for another use.) Place the three "steaks" on a cutting surface. Cut each cauliflower "steak" in half, cutting through the core so each piece looks like a fan (keep any pieces that come apart together).
Place 1 cup water in a very large nonstick skillet; bring to boiling. Add cauliflower steaks. Return to boiling; reduce heat. Simmer, covered, for 12 to 15 minutes or until just tender. Using a wide spatula, transfer steaks to a baking sheet or tray. Pat dry with paper towels. Cool slightly.
Place flour in a shallow dish. In another shallow dish beat together egg and milk. In third shallow dish stir together panko, salt, and pepper. Carefully dip each cauliflower steak in flour, egg mixture, then panko to coat both sides.
Drain any water from the skillet and wipe out skillet. Heat oil in the skillet over medium heat. Add coated cauliflower steaks; cook for 6 to 8 minutes or until golden and tender, carefully turning once.
To serve, spread bread slices with mayonnaise. Top with cheese, spinach, and cauliflower steaks. Top with Pickled Veggies.
In a small non-reactive bowl combine cauliflower, carrot, sweet pepper, and thyme. In a small saucepan combine vinegar, sugar, turmeric, and salt; bring just to boiling over medium-high heat, stirring to dissolve sugar. Pour immediately over vegetables, stirring to combine; cool slightly. Cover and chill for 1 hour or up to 3 days.
Before serving, drain vegetables, reserving liquid. Place drained vegetables and 3 to 4 tablespoons of the reserved liquid in a food processor. Cover and process until finely chopped.