Upside-down chicken pot pies

Ingredients

• 1/2 450-g pkg frozen pre-rolled puff pastry , thawed in fridge
• 1 very small red onion , cut into thin rings
• 1 red bell pepper , cut into thin rounds
• 1 tsp vegetable oil
• 2 skinless, boneless chicken breasts , cut into bite-sized pieces
• 3 tbsp butter
• 3 tbsp all-purpose flour
• 2 cups milk
• 1 cup frozen peas
• 1 cup broccoli florets
• 1/2 tsp salt
• 1 tsp lemon juice