Step 1

Place the pork bones into a large pot and cover with water. Bring to a boil over medium-high heat and cook at a rolling boil for 5 minutes. Drain the pork bones into a colander in the sink and rinse well until water against the bones runs clear; this is the most important step of the process.

Step 2

Turn on a multi-functional pressure cooker (such as Instant Pot®) and select Saute function. Warm the olive oil in the pot. Add leeks, onion, and garlic. Saute until onion is clear and has begun to brown, 7 to 10 minutes. Turn Saute mode off.

Step 3

Place the cleaned pork bones into the inner pot on top of the onion mixture. Add 4 cups of water. Place the lid onto the Instant Pot®, with the vent set to Sealing. Select high pressure according to manufacturer's instructions; set timer for 45 minutes. Allow 10 to 15 minutes for pressure to build.

Step 4

Release pressure using the natural-release method according to manufacturer's instructions, 10 to 40 minutes, or turn vent to Venting.

Step 5

Remove lid and carefully remove the inner pot. Strain the stock in a cheesecloth-lined colander that is placed on top of a large saucepan. Set aside. Remove any remaining meat from bones and set aside.

Step 6

Season strained broth with salt and pepper. Add in dashi and remaining 1 cup water. Stir in soy sauce. Bring broth to a slow boil over medium-low heat; turn heat down to a simmer. Scoop out 1/3 cup of the broth into a bowl. Add the miso into the bowl and "soften" the miso with a spoon, dissolving it into the broth. Return this paste to the saucepan. Mix paste into the broth and combine well. Add spinach to the broth. Continue simmering for 10 minutes (do not bring to a boil).

Step 7

Meanwhile, bring water to a boil in a pot. Add ramen noodles and cook until soft, about 2 minutes. Drain and rinse and place a good handful into a serving bowl. Pour tonkotsu broth into the bowl using a ladle. Add the pulled meat from the bones atop the noodles; add the spinach.