Preheat the oven to 400 degrees. Line a baking sheet with parchment paper or a silicone mat or spray it with nonstick cooking spray.
Toast the bread (if using).
Place the thawed spinach in a fine mesh strainer, squeeze out any excess water over the sink, and then place it in the food processor.
Chop and add the toasted bread or breadcrumbs, the cheese, egg, garlic, mayonnaise, mustard, salt and pepper. Pulse it until it is thoroughly combined.
Using wet hands, form the mixture into golf ball sized balls and put them on the baking sheet. (It should make about 20 balls.)
Bake them for 20 minutes until they are slightly puffed and lightly browned.
Meanwhile, make the dipping sauce (if using) by combining the sour cream, mayonnaise, dill and garlic. Add salt and pepper if desired, or 1 tsp. fresh lemon juice. Serve immediately.