In a large pot, heat the olive oil over medium heat. Add the diced onion and minced garlic, and sauté until they become translucent and fragrant.
Add the diced carrots, celery, bell pepper, zucchini, and mushrooms to the pot. Cook for about 5 minutes, stirring occasionally, until the vegetables start to soften.
Pour in the can of diced tomatoes and vegetable broth. Stir well to combine. Add the butter beans, dried thyme, dried rosemary, salt, and pepper. Bring the stew to a simmer and let it cook for about 20 minutes to allow the flavors to meld together.
While the stew is simmering, prepare the dumplings. In a mixing bowl, whisk together the all-purpose flour, baking powder, and salt. Gradually pour in the non-dairy milk and melted vegan butter. Stir until just combined, being careful not to overmix.
Make your cornstarch slurry by combining 2 tablespoon of cornstarch with 2 tablespoon of water. Stir it into the stew.
After the stew has simmered for 20 minutes and you've added the cornstarch slurry, drop spoonfuls of the dumpling batter onto the surface of the stew. Cover the pot with a lid and let the dumplings steam for about 15 minutes, or until they are cooked through and fluffy.
Once the dumplings are cooked, give the stew a gentle stir to incorporate them. Taste and adjust the seasoning if needed.
Serve the vegan stew with dumplings hot in bowls. Garnish with fresh herbs, such as parsley or thyme, if desired.