Select the Sauté setting on the Instant Pot (normal or medium heat level). After a few minutes, add the olive oil. Once the oil is hot, add the onion with a pinch of salt. Cook until the onion is nicely golden and completely soft, 6 to 8 minutes.
Add the carrots and garlic along with a pinch of salt and a few cracks of pepper. Cook until the carrots are just starting to soften, about 3 minutes.
Pour in a bit of the vegetable broth to deglaze the pot and scrape up any browned bits. Add the remaining broth, lentils, potatoes, bouquet garni, 1 1/2 teaspoons kosher salt, and several cracks of pepper. Stir gently.
Pour the crushed tomatoes (including juices) on top, but don’t stir (this prevents the tomatoes from sinking to the bottom and triggering the Burn notice). Note: getting the can nice and low right into the pot helps keep the tomatoes resting on top (instead of pouring from high above). Secure the lid and set the valve to Sealing (not Venting). Select the Pressure Cook setting (or Manual setting on older models) at high pressure and set the cook time to 12 minutes.
Once the 12-minute timer has completed and beeps, allow a natural pressure release for 10 minutes. Then manually release any remaining steam.
Open the pot and remove the bouquet garni. Stir in the dijon mustard and balsamic vinegar and taste. Add more salt and pepper to taste.This is a hearty and thick soup/stew, but if you want it thinner, add more broth until you reach your desired consistency and stir until warmed through.