Vegan Streusel Cake (German)

For the base:

• 1 Tablespoon chia seeds
• 1 Tablespoon cold water
• 100 ml plant-based milk
• 120 g apple sauce
• 2 Teaspoons lemon juice
• ½ Teaspoon vanilla extract
• 160 g oat flour
• 2 Teaspoons baking powder

For the streusel:

• 50 g oat flour
• 50 g almond flour
• 40 g agave nectar
• 1 Teaspoon cinnamon

For the filling:

• 500 g plums (pitted and chopped into small pieces)