Line a large baking sheet with a silicone liner or parchment paper.
Toss the onions and sweet potatoes in oil and salt and spread out on the prepared baking sheet. Roast for about 50 min, until the vegetables are cooked but not burnt. You don’t want any black bits. You are going to put them into the pot to boil and cook further.
Transfer the vegetables to a large saucepan and add the vegetable broth, ginger, turmeric, and cayenne.
Bring the mixture to a boil, and then reduce the heat to medium-high, and simmer, partially covered, for about 15 minutes to allow the flavors to infuse.
Remove the soup from the heat and stir in the macadamias. Allow the mixture to stand for 10 minutes to soften the nuts and cool the soup slightly.
Pour the soup into your blender in batches and puree on high for 1 to 2 minutes, until smooth and creamy. (For conventional blenders, remove the small center lid cap and cover the opening with a kitchen towel so steam can escape while you blend.)
Return the soup to the saucepan, season to taste, and warm it over low heat.
To serve, ladle the soup into bowls and add a splash of the lime juice, top with cilantro and macadamias if using.