Peel and core the quince.

Place in a large saucepan and cover with water.

Add the lemon juice and sprinkle 3 tablespoons of sugar over the top.

Bring the water to a boil over medium-high heat.

Reduce the heat to low. Simmer until tender but not mushy. About 1/2 hour.

Gently drain. When cool enough to handle, slice each quarter into four pieces. Set aside.

Preheat the oven to 350F.

Line the bottom and sides of a 10-inch springform pan with greased parchment paper.

Melt the butter. Let cool until just warm to the touch.

Whisk together the brown sugar, all-purpose flour, salt, and cocoa powder.

Stir the vanilla into the melted butter. Add to the dry ingredients.

Mix until large streusel crumbs form.

Press 2/3 of the chocolate streusel into the pan and halfway up the sides.

Sprinkle the rolled oats over the top of the streusel crust.

Arrange the poached quince slices over the top gently pressing them in.

Sprinkle the rest of the streusel over the top. Gently press it down with your hands.

Bake for 45 minutes. Check the quince after 30 minutes with a fork to make sure it is tender.

If the quince is starting to brown, cover the pan with aluminum foil for the remaining 15 minutes.

Let cool for 15 minutes before de-panning.

Line a baking sheet with parchment paper and place a cooling rack on top.

Transfer the cake to the cooling rack. Let cool completely.

In a double boiler melt the bittersweet chocolate over low heat.

Let cool enough to handle.

Spoon or scrape the chocolate into a pastry bag fitted with a #4 tip.

Squeeze the chocolate over the top of the cake in a zig-zag pattern.

Or, use a fork or spoon to drizzle the chocolate over the top of the cake.

Transfer to a platter. Cut and serve.