Warm the oil in a large pot, add onions & garlic, cover and cook on medium heat for 10 to 12 min, stirring occasionally. Add the salt, thyme, marjoram, shrooms and mustard. Cook till the shrooms start to release their juices, about 5 minutes.
Add the sweet potatoes, white potatoes, parsnips, black pepper and water, bring to a boil. Reduce heat, cover and simmer for 15 - 20 min, until the vegetables are tender. Stir the dissolved corn starch mixture into the simmering vegetables, stirring constantly. When the liquid starts to thicken, mix in the peas, corn, soy sauce and salt. Pour the vegetables into the prepared dish and set aside.
In a mixing bowl, sift together flour, salt, baking powder and baking soda. In a separate bowl mix together the melted butter and buttermilk or yogurt. Combine the wet and dry ingredients in as few strokes as possible to make a soft dough. Drop the biscuit batter over the vegetables in the casserole dish in six equal mounds. Sprinkle the dill over the biscuits.
Bake for 25 to 30 minutes or until a knife inserted into the center of a biscuit comes out clean. Serve immediately.