Fill large pot or other large, heat-resistant container with water. Insert sous vide immersion circulator and set to 165° Fahrenheit. Note: pot must be deep enough that water level falls between minimum and maximum indicators on circulator.
Add soy sauce, honey, and lemon juice to small mixing bowl. Whisk ingredients together until fully combined.
Place pork belly in food-safe plastic bag. Pour marinade mixture into bag and toss or flip pork belly as needed to completely coat in marinade.
Remove as much air from bag as possible with vacuum sealer or water displacement method (see Notes) and seal bag completely. Place sealed bag in preheated water bath, making sure to submerge pork belly completely.
Cook pork belly, fully submerged in water, 10 hours. Note: Food must be fully submerged to cook properly. If food won't submerge on its own, attach pie weights or silverware to bottom of bag for extra weight.
After 10 hours, remove bag from water bath and set aside.
Heat large skillet over medium-high heat. Once skillet is warm, add oil and swirl pan to distribute oil evenly across surface. Continue heating skillet until oil is hot and shimmery.
When oil is hot, transfer pork belly from bag to skillet. Sear 1 to 2 minutes or until golden, then flip pork belly slices over and sear 1 to 2 minutes more.
Transfer seared pork belly to serving plate. Garnish with sesame seeds and sliced green onions if desired and serve warm with preferred sides.