Turn on the Instant Pot and select the sauté function. Add the olive oil and allow it to heat up. Once hot, add the smashed garlic cloves and sauté for about 1 minute, until fragrant.
Next, toss in the chopped baby spinach. Stir for about 2-3 minutes until the spinach wilts down.
Add the water and Kosher salt to the pot. Then, pour in the whole wheat pasta and gently mix everything together.
Pour the marinara sauce over the pasta and spinach mixture. Do not stir; just layer it on top.
Seal the Instant Pot lid and make sure the pressure valve is set to sealing. Select the manual or pressure cook option and set the timer for 6 minutes.
Once the cooking time is up, perform a quick release of pressure. Carefully open the lid, and give everything a good stir. Add the ricotta, grated Pecorino Romano, and mozzarella cheese. Gently fold the cheese into the pasta until well combined.
Spoon the Instant Pot Baked Ziti into bowls and enjoy it hot. You can top it with extra cheese or fresh herbs if desired.