Preheat oven to 350°F. Lightly butter and flour the loaf pan, knocking out any excess flour.
In a small bowl, mix together the chia seeds and water. Let them sit for 10 minutes to soften the chia seeds.
Next, make your streusel topping. Combine the streusel ingredients in a bowl and use your fingers to incorporate the butter, letting the mixture clump. Set aside. Using a stand mixer, make the batter for the banana bread. Add the butter and blend until light and creamy. Slowly add the sugar and beat until fluffy, about 2 minutes. Add the eggs, one at a time, mixing well after each addition. Add the mashed bananas and beat until the mixture is smooth. Scrape down the sides of the bowl with a rubber spatula. Mix in the sour cream, vanilla, lemon juice, and chia-water mixture.
In another large bowl, stir together the almond flour, brown rice flour, oat flour, baking soda, salt, walnuts, and cinnamon. Add to the banana mixture, beating just until combined. Transfer the mixture to the prepared loaf pan and top evenly with streusel.
Bake for about 1 hour, until a cake tester inserted in the center, comes out clean. Let cool before removing the loaf from the pan. Serve the bread at room temperature.