First, make the sauce. In a medium saucepan, combine all ingredients for the sauce and bring to a boil over medium-high heat. Cover the pot, reduce the heat to low, and allow to simmer for 10 to 12 minutes, until the rhubarb is broken down and the berries are softened. Remove the vanilla bean and use a potato masher to mash the strawberries: the sauce will be textured. If a smoother texture is desired, transfer to a food processor or blender and process until smooth. Set aside until needed, or refrigerate if using another day.
To a make the pancakes, whisk together the ground flaxseed and warm water in a small bowl and set aside for 5 to 10 minutes to gel. This flax “egg” will help the pancakes hold together.
In a medium bowl, whisk together the buckwheat flour, baking powder, sugar, and salt. In a smaller bowl, whisk together the non-dairy milk, oil, and flax “egg”. Add the wet ingredients to the bowl of dry ingredients and mix until well-incorporated.
Heat a nonstick skillet or griddle over medium-high heat until hot. To test the heat, flick a drop of water onto the surface of the skillet: when it’s hot enough, the water will evaporate almost immediately. Brush or spray the skillet lightly with oil, then measure out ¼ cup batter. (I like to use a dry ¼-cup measuring cup for this.) Use the bottom of the measuring cup to gently spread the batter out until the pancake is roughly 3 to 4 inches across. Repeat until you run out of skillet space.
Cook the pancakes for 1½ to 2 minutes, until the edges look dry and you can see bubbles surfacing. Carefully flip the pancakes and allow to cook for an additional minute or so, until they’re puffy and feel firm when pressed. Transfer the finished pancakes to a plate and repeat until the batter is gone. Serve the pancakes with strawberry-rhubarb sauce, or with vegan butter and maple syrup.