For the crust, whisk together flour, cornmeal, 1½ teaspoons salt and baking soda. Combine avocado, sour cream and 2 tablespoons milk. Stir avocado mixture into flour mixture with a fork. If the dough looks too dry, add more milk, one tablespoon at a time, until it holds together when mixed. Turn dough out onto a work surface and lightly knead in any loose flour. Divide dough into 2/3 and 1/3 amounts for the top and bottom crusts; wrap each in plastic wrap, flatten into discs and chill at least 30 minutes.