Preheat the oven to 400 degrees farenheit
Put the brussels sprouts and sweet potato pieces in separate bowls. Roll each group in 1 tbsp of olive oil (maybe a bit less for the brussels sprouts) and 1 tsp of sea salt in their respective bowls.
Spread the sweet potatoes on a baking sheet lined with parchment paper and bake in the oven for 23 minutes. This will not fully cook them, but they will be added back with the other ingredients soon.
Do the same with the brussels sprouts and bake for 10 minutes.
Let the hot brussels sprouts and sweet potatoes cool to a comfortable level to touch.
Add the brussels sprouts, sweet potatoes, apples, and carrots to a large bowl (or two) and mix with 1½ tbsp of olive oil and all of the cinnamon, cardamom, nutmeg, and ½ tsp of sea salt. Then, use two baking sheets lined with parchment paper to bake for 12 more minutes.
Let the hash cool and top with walnuts before serving