Preheat the oven to 425°F. Toss the potatoes with 1 tablespoon olive oil, ½ teaspoon salt, and ¼ teaspoon black pepper. Spread onto a parchment-lined baking sheet and roast for 20-25 minutes, or until beginning to brown and almost fork-tender.
Meanwhile, heat the remaining 1 tablespoon olive oil in a large skillet over medium heat. Add the onion and bell pepper and cook for 3-4 minutes, until slightly softened. Add the tempeh bacon and cook for another 3-4 minutes, or until lightly browned. Stir in the spinach and cook just until wilted. Season with the remaining ½ teaspoon salt and ¼ teaspoon black pepper.
Add the drained cashews, water, nutritional yeast, cornstarch, lemon juice, Dijon mustard, garlic powder, onion powder, smoked paprika, salt, pepper, and turmeric to a high-speed blender. Blend until completely smooth and creamy.
Reduce the oven temperature to 375°F and lightly grease a 9x13-inch baking dish. Add the roasted potatoes and veggie-tempeh mixture. Pour the cashew filling evenly over the top and add 2 tablespoons of the dill. Gently stir until everything is evenly coated and distributed.
Bake uncovered for 25-30 minutes, or until the filling is set around the edges and the center is no longer liquid.
Let the casserole rest for 15-20 minutes to allow the filling to set. Sprinkle with the remaining fresh dill, then slice and serve.