My Grandmother's Rhubarb Flapjack

Ingredients

• 2 cups (200g) rolled oats
• 1 cup (140g) all-purpose flour
• 1/2 cup (100g) packed brown sugar
• 1/2 cup (120ml) unsalted butter, melted
• 1/4 cup (60ml) golden syrup or honey
• 1 teaspoon (5g) baking powder
• 1/4 teaspoon (1g) salt
• 2 cups (250g) rhubarb, chopped into small pieces
• 1/4 cup (50g) granulated sugar
• 1 tablespoon (15ml) lemon juice