For dough: In a bowl, mix masa flour, sugar, baking powder, and salt. Add cream-style corn, corn kernels, chilies, white onion, and oil; mix until blended.
For spinach filling: In a 10- to 12-inch frying pan over medium-high heat, stir cumin seeds, coriander seeds, and oregano in oil until fragrant, about 30 seconds. Add onion, bell pepper, and garlic; stir occasionally until onion is limp, about 3 minutes. Press liquid out of spinach. Add spinach and tomatoes to pan and stir often until liquid has evaporated, 2 to 3 minutes. Add salt to taste.
Lightly coat insides of eight ramekins or custard dishes (3/4 to 1 cup each) with cooking oil spray (to wrap tarts in cornhusks, see "Party Tarts," below.) Spoon an eighth of the dough (a scant 1/2 cup) into each dish. With the back of a spoon, spread dough over bottom and partially up sides of dish; it should be about 1/4 inch thick. Evenly layer an eighth of each of the pepper cheese, spinach filling, and Mexican cheese blend over dough. Arrange dishes on a 10- by 15-inch baking pan. Cover pan with foil.
Bake in a 350° oven until tarts are hot in the center, about 30 minutes. Serve with salsa.
Party Tarts: You can bake the savory treats directly in ramekins or dress them up in husks. You'll need about 35 dried cornhusks (each 4 in. wide; 2 1/2 oz. total; available at Mexican markets or well-stocked supermarkets). Make dough and filling for tamale tarts (through step 2 above), then prepare the husks.
Separate husks; discard silks. Soak husks in hot water until pliable, about 20 minutes. Rinse well and drain. Tear with grain into sections 2 to 3 inches wide at large end.
Overlap husk pieces to line eight baking dishes (about 4 in. wide), wide ends in bottom, narrow ends over sides.
Fill each with dough; spread and layer (step 3 above).
Leave open, or tie into packets: Tear more husks, with the grain, into 1/4- to 1/2-inch-wide strips (if strips are shorter than 8 inches, tie ends of two short strips together). Gather husks over filling in each dish; tie with a strip.
Set open-faced tarts or packets in a baking pan, cover, and bake (steps 3 and 4 of above recipe).