Preheat the oven to 350F and grease your doughnut pan.
If using whole oats, pistachios, almonds, and/or flax seeds, do the following: Grind 1/2 cup oats into a fine flour in a food processor. Then grind 1/4 cup pistachios into a fine meal in a coffee grinder or food processor, and do the same with 1/4 cup almonds and 2 tablespoons flax seeds (separately).
Combine the oat flour, pistachio meal, almond meal, and flax meal along with 1/4 cup sweet rice flour, 1 teaspoon baking powder, and 1/2 teaspoon salt in a large bowl, mixing well. In a separate bowl, whisk the egg. Then whisk in 1/2 cup milk, 1/4 cup honey, 1/4 cup applesauce, 1 tablespoon oil, and 1 1/2 teaspoons vanilla extract.
Pour the wet mixture into the dry ingredients and stir with a large wooden spoon until just combined, being careful not to overmix (stop when you no longer see dry flour).
Spoon the batter into the doughnut molds, filling to just below the top of each mold. Bake for 18 to 22 minutes until lightly golden brown around the edges. A toothpick inserted in the center should come out clean. Let cool in the pan for 5 minutes. Slide a thin spatula around the edges of the doughnuts to help loosen them out. Then place on a cooling rack and allow to cool fully before topping.
To make the topping, chop 3/4 cup pistachios. In a bowl, mix the pistachios and salt together (if using unsalted nuts). In a microwavable bowl, add 1/4 cup honey, and microwave about 20 seconds until hot. Place parchment paper under the cooling racks, then generously spoon honey over the tops of the doughnuts. Top each doughnut with ground pistachios. Let cool and serve.