Preheat the oven to 500˚. Combine the spinach, artichokes, cream cheese, parmesan, mayonnaise, sour cream, garlic, lemon zest, 1/2 teaspoon salt and a few grinds of pepper in a food processor. Puree until very smooth.
Put the bagels cut-side up on a rimmed baking sheet. Bake until lightly toasted, about 5 minutes. Spread a heaping 1/4 cup of the spinach mixture on each bagel half. Dot each with 3 small spoonfuls of ricotta, then sprinkle with the mozzarella. Bake until the cheese is melted and bubbling, 10 to 12 minutes.
Top the bagels with more parmesan and a pinch of red pepper flakes.
Be sure to squeeze the excess water from your spinach and artichokes — it will keep the bagels from getting soggy.