Preheat the oven to 350°F (175°C) and grease an 8-inch or 9-inch square baking pan.
In a medium mixing bowl, whisk together the flour, baking soda, salt, ginger, cinnamon, cloves, and nutmeg.
In a large mixing bowl, whisk the Greek yogurt, molasses, maple syrup, eggs, and melted coconut oil until well incorporated.
Gently fold the dry ingredients into the wet ingredients using a spatula until just combined; do not overmix.
In a small bowl, combine the rolled oats, chopped walnuts, coconut sugar, and 1/2 teaspoon of cinnamon for the streusel.
Pour the batter into the prepared baking pan and spread it evenly.
Sprinkle the oat and walnut streusel mixture over the top of the batter.
Bake for 30 to 35 minutes, or until a toothpick inserted into the center of the cake comes out clean.
Remove from the oven and allow to cool on a wire rack for at least 15 minutes before serving.