Beef rib roast with spiced salt and roasted onions

Ingredients

• 1 tbsp coriander seeds
• 1 tbsp cumin seeds
• 1/2 cup (60g) sea salt flakes
• 1/4 tsp mixed spice
• 1 tsp freshly ground pepper
• 3 kg beef standing rib roast (about 4 ribs)
• 1 tbsp olive oil
• Roasted onions
• 2 large brown onions, halved crossways*
• 4 small red onions, quartered
• 2 bulbs garlic, halved crossways
• 1/4 cup olive oil
• Garlic mayonnaise
• 1 cup whole egg mayonnaise
• 1 tbsp water
• 2 tsp lemon juice
• 2 cloves garlic, crushed
• 1 tbsp dijon mustard