Preheat the oven to 425°F. Slice the sweet potatoes in half lengthwise, transfer to a baking sheet. Rub sweet potatoes with olive oil, season with salt and pepper and place cut side down and roast until tender, 25 to 30 minutes.
Drain and rinse the chickpeas. Halve and juice the lemon into a small bowl. In a large bowl, combine the chickpeas, sliced cucumbers, parsley leaves, 1 tbsp lemon juice, olive oil, and a pinch of salt. Toss cucumber and chickpea mixture to combine.
In a medium bowl, combine the white miso paste, white vinegar, 1 tsp of lemon juice, and 2 tbsp water. Whisk until smooth.
Slice the gai lan and chop it into bite-size pieces. Heat a large nonstick skillet with 1 tsp of olive oil over medium-high heat. Add the chopped gai lan, 2 tbsp of water and cook until bright green and crisp-tender, 2 to 3 minutes.
Remove the roasted sweet potatoes from the oven. Stuff them with the cooked gai lan and cucumber chickpea salad. Sprinkle with sesame seeds and red chili pepper flakes if desired.