Preheat oven to 350ºF. Line a 9" loaf pan with parchment paper, leaving the edges to hang over the side (this'll make it easier to remove the loaf when it's done).
Mix together the nondairy milk and vinegar, stir well, and set aside to let curdle (this is a homemade vegan buttermilk!)
Mix the flaxseed meal with 6 Tbsp. of water, mix well, and set aside to thicken (about 5-7 minutes).
Sift together the dry ingredients (flour, cinnamon, salt, baking powder, and baking soda). Add the brown sugar and mix well to make sure there are no clumps.
Add the wet ingredients (buttermilk, flax eggs, and vanilla extract) and mix until smooth.
Pour 1/3 of the mixture into the loaf pan and spread evenly. Top with half of the jam, spreading almost to the edges (be sure to leave a small border so the jam doesn't ooze out the sides). Repeat once more, and top with remaining 1/3 of batter.
To make the streusel: sift together the dry ingredients (flour, cinnamon, and salt). Add the sugars and mix well to make sure there are no clumps. Add the chopped walnuts, then mix in the cold cubed butter and press the mixture together until it resembles small crumbs. Spread evenly across the top of the batter.
Bake for 30 minutes, then remove from oven and cover with foil. Place back in the oven and bake an additional 45 minutes until golden brown and fully baked all the way through.
Once the loaf is fully cooled, mix together the glaze ingredients and drizzle over the top of the loaf.