Beet and black bean burger with avocado mayonnaise - EAT ME

Ingredients

• 2 red onions
• 125 ml vinegar
• 1 tbsp caster sugar
• 100 g oat flakes
• 400 g mini beetroot
• 1 tin of black beans (400 g), drained
• 2 cloves of garlic
• 3 tbsp olive oil
• 1½ tsp smoked paprika powder
• 1 tsp ground cumin
• 1 lime
• 2 avocados
• 3 tbsp (vegan) mayonnaise
• 6 gherkins
• 6 hamburger rolls
• 50 g spinach