Preheat the oven to 400F. Bonus tip: add your baking sheet into the preheating oven so it gets hot and the sweet potato doesn't stick.
In the meantime, peel and cube the sweet potato, add to a shallow bowl and season with avocado oil, all spice, sea salt, and pepper. Work seasoning into the sweet potato cubes with your hands. Then add seasoned cubes to the hot baking sheet and place back in the oven for 20-25 minutes stirring midway.
In the meantime, add lettuce, finely chopped onion, finely chopped dill, coarsely chopped walnuts, dried cranberries, and crumbled feta cheese to a large salad bowl.
Prepare the dessing by adding Dijon mustard, apple cider vinegar, maple syrup, and olive oil to a sealable jar and shake until well combined.
About 5 minutes before the sweet potato will be done roasting, cut 1/4 avocado into dice and add to the salad as well.
Add roasted sweet potato to the salad bowl, drizzle dressing over everything and give a good toss.