Steamed veg with sautéd onion, garlic, lemon and tarragon dressing

Name: Steamed veg with sautéd onion, garlic, lemon and tarragon dressing
Cuisine: French
Ingredients: A glug of olive oil; 1 finely chopped onion; 1 crushed garlic clove; Finely grated zest and juice of 1 lemon; A bunch of roughly chopped fresh tarragon; 5 tbsp extra-virgin olive oil; 500g sliced leeks; 1 broccoli, separated into florets (slice thick stems); 200g frozen peas