Preheat the oven to 400°F.
Cut the sweet potatoes into matchsticks, being careful to cut into evenly sized pieces. The thinner you slice the potatoes the faster they will cook.
Place the potatoes on a baking sheet lined with parchment paper or foil. Drizzle with 1 tablespoon of the olive oil and ½ teaspoon of the salt, tossing to coat. Be careful not to overcrowd the pan. If the potatoes are overlapping use a second pan.
Bake the sweet potatoes for 20-30 minutes, flipping halfway through. Remove from the oven when the potatoes are lightly browned and tender all the way through. They may not be super crispy, but as long as they're tender and browned they'll be perfect in this recipe.
While the potatoes cook, heat a medium skillet over medium heat. Add 1 tablespoon of olive oil, the onion, and ¼ teaspoon of salt. Sauté for 5-7 minutes, stirring occasionally, until the onions are soft.
Turn the heat to medium-high, add the beef, ¼ teaspoon of salt, cumin, garlic powder, and chili powder. Sauté for 5 minutes, stirring occasionally, until beef is browned and cooked through.
To serve, place sweet potatoes on a plate. Top with lettuce, the cooked beef mixture, tomato, guacamole or avocado, and scallions.