Add the vegan butter (or olive oil) to a large pot or Dutch oven and heat over medium heat. Once melted, add the chopped onion and sauté for a few minutes until slightly softened. Then add the sliced mushrooms, dried thyme, dried rosemary, sea salt, and black pepper. Cook for 7-10 minutes, stirring occasionally, until the mushrooms have released most of their moisture and start to brown. Add the minced garlic and cook for another minute until fragrant.

Sprinkle the cornstarch over the sautéed vegetables and stir to coat evenly. Pour in the tamari (or soy sauce) and Dijon mustard and mix well to combine. This helps to build the flavor base and prepares the vegetables for simmering. I like to make sure everything is well coated before adding the liquids for a richer stew.

Add the vegetable broth and baby potatoes to the pot. Increase the heat to bring the mixture to a boil. Once boiling, reduce the heat and simmer uncovered for 15-20 minutes, or until the potatoes are tender. Stir occasionally to prevent sticking and to distribute the heat evenly.

Stir in the rinsed and drained white beans and nondairy milk with the simmered mixture (from Step 3). Continue simmering uncovered for another 10-15 minutes, until the vegetables are fully tender and the stew has thickened. Adjust seasoning and the liquid amount as needed: more tamari or salt for saltiness, more mustard for acidity, or more nondairy milk if you'd like a thinner consistency. I like to taste and tweak the flavors here to make the stew just right.

Serve the stew warm, garnishing each bowl with chopped fresh parsley and a sprinkle of freshly cracked black pepper to taste. I sometimes add a squeeze of lemon juice right before serving for a touch of brightness. Enjoy your comforting vegan mushroom and potato stew!