Preheat oven to 425F. Line a couple rimmed baking sheets with parchment.
In a large bowl, toss the potatoes with the avocado or sunflower oil and a generous pinch of salt. Spread onto the baking sheets in a single later. Roast for 28-35 minutes, until the potatoes are tender and golden brown. Place the potatoes in a large mixing bowl. Set aside to cool.
Place the garlic, avocado, 6 tablespoons of olive oil, 3 tablespoons of lemon juice, dill, a pinch of salt, and black pepper in the bowl of a food processor. Pulse until the avocado and dill are chopped and combined, but leave it a little lumpy. Taste, and add up to 2 more tablespoons each of avocado and lemon juice and more salt and pepper as needed.
Add the avocado-dill mixture to the cooled potatoes and toss to coat. Taste for seasoning, and serve sprinkled with more fresh dill.