Heat the oven to 425?F. Poke the sweet potatoes a few times with a fork and place on a sheet pan. Roast until fork-tender, 35 to 45 minutes. Alternatively, steam in a medium saucepan with a few inches of water until fork-tender, about 35 minutes. Set aside to cool.

Zest the lemon into a small bowl, then whisk in the tahini and 3 to 5 tablespoons of water until the mixture becomes smooth and thick. Season with salt and pepper and set aside. Halve the lemon.

In a large bowl, toss the green beans with 1 tablespoon of the olive oil. Heat a large cast-iron skillet over medium-high and add the beans. Cook, tossing occasionally, until charred, 4 to 6 minutes. Transfer back to the bowl, and toss in the grated garlic. Juice the lemon over the beans, season with salt and pepper, and set aside.

Heat the remaining 2 tablespoons of olive oil in the same skillet over medium-high. Slice or tear the sweet potatoes into irregular 2-inch pieces and cook until well browned all over, 5 to 8 minutes. Remove from heat and season with salt and pepper.

When ready to serve, smear the tahini mixture onto a large serving plate. Scrape the beans onto the plate and arrange the sweet potatoes on top. Sprinkle with flaky sea salt and chile flake if using.