Using a large cheese grater, grate the cold butter on to a cutting board. Put the board in the freezer until you are ready to make the dough.
Soak the cashews in the just-boiled water. Set aside for at least 30 minutes.
Combine the flour, salt & pepper in a food processor and pulse to combine.
Remove the grated butter from the freezer and gradually add it to the food processor. Continue to pulse until well combined. Slowly add the ice water 1 tablespoon at a time while pulsing until a dough forms. Wrap the dough in a small piece of plastic wrap and chill in the refrigerator for ~30 minutes.
While the dough is chilling, remove the core and thickly slice the heirloom tomatoes. Lay the slices on a paper towel covered plate and sprinkle with 1 t. of salt to draw out the moisture.
Preheat the oven to 400º. While the tomatoes are resting, slice the onions and mince the garlic. Heat the olive oil in a small skillet. When shimmering, add the sliced yellow onions. Turn the heat down to a low setting and allow the onions to gently brown.
Remove the dough from the refrigerator and roll it out on a lightly floured surface. Press into a pie plate and trim edges. Cover with a layer of parchment paper and fill with dried beans to weigh the crust down. Pre-bake the pie crust for 12 minutes and then allow the crust to cool for ~10 minutes. Remove the beans.
When the onions have turned golden brown, add the garlic and cook on low for 1-2 minutes more. Set aside.
Pat the tomatoes dry.
Drain and rinse the cashews. Combine the cashews, water, garlic, salt, lemon juice, nutritional yeast and flaxseed meal in a high-speed blender. Blend until creamy. The cashew cream sauce should be quite warm when it comes out of the blender.
Pour the cashew cream sauce into the pre-baked pie crust. Add a layer of sliced tomatoes, onions and garlic. Season with salt and pepper and bake the tomato pie for 20-25 minutes. When 5 minutes remain, add the basil leaves to the top of the tomato pie and finish baking.
Remove the pie from the oven and allow to cool for 10 minutes before serving.