Let ribs come to room temperature on the counter for 30 minutes.

Pat ribs dry and season with Chinese 5 spice, kosher salt, and black pepper.

Heat olive oil in a large stock pot over medium-high heat until shimmering.

Sear ribs in a single layer until browned on all sides, about 9-10 minutes total (3 minutes per side).

Remove ribs from the pot and transfer to a slow cooker.

Add sake to the pot and scrape up the browned bits from the bottom.

Cook sake until reduced by half, about 1-2 minutes.

Pour reduced sake over the ribs in the slow cooker.

Combine soy sauce, apple juice, dark brown sugar, sesame oil, rice wine vinegar, green onion, garlic, ginger, and red pepper flakes in a bowl.

Add the sauce mixture to the slow cooker.

Cover the slow cooker and cook on high for 5-7 hours, or on low for 9-11 hours, until ribs are fall-off-the-bone tender.

Turn/flip ribs over 2 or 3 times during cooking.

Carefully remove ribs from slow cooker and transfer to a serving platter or clean work surface.

Cover loosely with aluminum foil.

Strain the cooking sauce and let fat separate (use a fat separator or wait a minute for fat to rise to the top). Discard fat.

Transfer sauce to a small saucepan over medium/low-medium heat.

Cook sauce for 10-12 minutes or until reduced by half.

In a small bowl, mix cornstarch and water to make a slurry.

Whisk slurry into sauce and continue to cook for an additional 2 minutes, or until thickened.

Pour or brush sauce over ribs, reserving some to serve on the side.

For the lettuce wraps: Use two forks to remove meat from bone and shred.

Divide sushi rice, cucumber salad (optional), and meat among lettuce cups.

Spoon some of the additional sauce over the lettuce wraps and garnish with sesame seeds.