Preheat the oven to 350°F/180°C (convection). Line a baking sheet with parchment paper.
Slice the sweet potato into ½-inch/1.25 cm rounds and arrange in a single layer on the sheet. Brush both sides with olive oil and sprinkle with ½ teaspoon salt. Roast until tender and golden at the edges, 20 to 25 minutes. Let cool slightly.
While the sweet potatoes roast, mash the avocado with the lemon juice and ¼ teaspoon salt until mostly smooth. Fold in the red onion, diced cherry tomatoes, and chopped cilantro.
Spread a spoonful of guacamole over each warm sweet potato round. Top with crumbled feta and extra cilantro leaves. Serve immediately.