Preheat your oven to 400°F and line a baking sheet with parchment paper for easy cleanup.
Peel and dice the sweet potato into half-inch cubes, then toss them on the baking sheet with 0.5 tbsp olive oil and half of the salt and pepper.
Roast the sweet potatoes for 20-25 minutes, flipping halfway through, until they are golden brown and tender.
While the potatoes roast, season the chicken breast evenly with the dried oregano, garlic powder, and the remaining salt and pepper.
Heat the remaining 0.5 tbsp olive oil in a skillet over medium-high heat and sear the chicken for 6-7 minutes per side until the internal temperature reaches 165°F.
In a small bowl, whisk the lemon juice with a teaspoon of water to create a light, bright dressing.
Toss the mixed greens and sliced cucumber with the lemon dressing in a medium bowl until well coated.
Slice the chicken breast into strips and serve immediately alongside the roasted sweet potatoes and the fresh garden salad.