Preheat the oven to 390F (200C).
Grease a 13 x 9 X 2-inch (33 x 23 x 5 cm) baking pan. Line it with parchment paper.
In a small bowl, make the flaxseed-egg: whisk the flaxseed meal with 3 tablespoons of lukewarm water; let rest for 10 minutes.
Peel the potatoes and cook them until fork tender. Drain and let it cool for five minutes until it is safe to handle. Rice them in a large bowl. Add the flaxseed-egg, almond milk, all-purpose flour, 1 teaspoon of salt, and baking powder and mix with a wooden spoon until well combined, about one minute. Do not overmix. Pour it into the greased and lined baking pan, smooth it out with a spatula, and bake for 20 minutes.
While baking the potato, make the filling. In a large skillet over medium heat, sauté garlic in olive oil for 30 seconds or until fragrant. Add the spinach, one teaspoon of salt, and black pepper, and sauté for 8 minutes or until cooked down on its juice, and all liquid has evaporated. Remove from stove. Add walnuts and stir to combine.
Remove potato from oven and let it cool for 10 minutes until safe to handle. Flip it over on a large baking sheet or dry kitchen towel. Peel the parchment paper and discard. Spread the spinach filling evenly on the potato sheet. Starting from the shorter side, roll and place it in a serving dish keeping the seam in the bottom of the roulade. Take a skinny slice off each side to trim.