Preheat the oven to 350°F (175°C). Butter an 8- or 9-inch square pan (or a 9-inch round) and line the bottom with parchment for easy removal. Preheating ensures even rise; lining prevents sticking and makes an intact cake easier to unmold.

Prepare the oat-cinnamon streusel: in a medium bowl combine 1/2 cup (45 g) rolled oats (from the measured cup), 1/3 cup (70 g) packed light brown sugar, 1/4 cup (30 g) all-purpose flour (from the measured amount), 1 teaspoon ground cinnamon, a pinch of salt, and 3 tablespoons (42 g) cold unsalted butter, cut into small pieces. Rub the butter into the dry ingredients with your fingertips until you have coarse crumbs (pea-sized pieces). Mix in 1 tablespoon chia + 1 tablespoon ground flax for extra crunch. Making the streusel cold and coarse gives you crisp clusters after baking; the little butter pockets melt and create that desirable crunchy contrast to the soft cake.

Toss the diced apples with 1 teaspoon cinnamon and 1 tablespoon granulated sugar in a small bowl. This quick maceration pulls out some apple juices so the fruit will be juicy inside the cake but not sink and make the batter soggy.

In a large bowl, whisk together the remaining 1 cup (120 g) rolled oats (if any left), 1 1/4 cups (150 g) all-purpose flour (remaining from total), 3/4 cup (150 g) granulated sugar, baking powder, baking soda, 1 teaspoon cinnamon, 1/4 teaspoon nutmeg and 1/2 teaspoon salt. Sifting is optional, but whisking aerates the flour and evenly distributes leavening, which gives a lighter crumb.

In a separate bowl, beat together the eggs, applesauce, melted coconut oil, 6 tablespoons (85 g) softened unsalted butter (reserve 2 tablespoons for the streusel as part of the 113 g total), milk and vanilla until smooth. The applesauce replaces some fat and keeps the cake tender while adding natural apple flavor; beating helps emulsify fat and liquids so the batter holds air and rises evenly.

Pour the wet ingredients into the dry and fold gently with a spatula just until combined — a few streaks of flour are fine. Overmixing develops gluten and can make coffee cake tough; folding keeps the texture tender. If the batter seems very thick, add up to 2 tablespoons more milk.

Fold in the diced, macerated apples and the raisins. The raisins add pockets of concentrated sweetness and chew; folding them last keeps them distributed without overworking the batter.

Spread half the batter into the prepared pan and level with an offset spatula. Sprinkle half the streusel evenly over the batter, then dollop the remaining batter on top and spread. Finish with the remaining streusel and an optional scattering of coarse sugar or a few extra oats for visual crunch. Layering batter and streusel creates pockets of crumb throughout the cake rather than only on top.

Bake at 350°F (175°C) for 40–45 minutes, rotating the pan halfway through. Insert a skewer into the center — it should come out with a few moist crumbs but not wet batter. Baking until just done ensures a moist interior and crisp streusel; overbaking dries the cake.

While the cake bakes, make the orange-marmalade coffee glaze. In a small saucepan combine 3 tablespoons orange marmalade and 2 tablespoons hot brewed coffee. Warm over low heat, whisking until smooth and slightly reduced (1–2 minutes). Remove from heat and stir in 1 teaspoon cold butter to give shine and balance the acidity. Warming the marmalade mellows its bitterness and the coffee adds depth; finishing with butter emulsifies the glaze and gives a glossy sheen.

When the cake comes out, let it cool in the pan on a rack for 10–15 minutes. While still warm, brush the marmalade-coffee glaze over the top so it soaks into the crumbs and adds flavor without making the streusel soggy. Cooling slightly helps the cake set so it slices cleanly, but glazing while warm helps the flavor penetrate.

Transfer the cake to a rack to cool an additional 20 minutes before slicing into 8 squares. Serve slightly warm or at room temperature with a cup of coffee or tea.