Place the oven racks in the upper and lower thirds of your oven and coat two rimmed baking sheets with nonstick spray. Preheat the oven to 400 degrees F.
Place the cauliflower, garlic, and onions in a large bowl. Drizzle with olive oil and sprinkle with 2 tablespoons taco seasoning. Toss to coat, and then divide evenly between the two baking sheets. Spread each into a single layer so that the veggies do not overlap. Roast until golden and tender, about 30 minutes, tossing once halfway through and switching the pans’ positions on the upper and lower rack.
Transfer the roasted vegetables to a blender, and add in the soaked cashews, almond milk, nutritional yeast, salt, and remaining 1 1/2 teaspoons taco seasoning. Blend until very smooth, adding water a few tablespoons at a time as needed to reach your desired consistency. (I added about 1/2 cup water for a very thick queso.) If your blender has a heating feature, continue blending until the mixture is hot. If not, pour the queso into a saucepan and heat over medium on the stove until very warm. Stir in the drained tomatoes or salsa. Taste and add kosher salt or a little bit of additional taco seasoning as desired.
To serve as a dip, transfer to a serving bowl and sprinkle with desired toppings. For nachos, drizzle the queso generously over a bed of tortilla chips and garnish as desired (place in the oven for a few minutes to make extra warm if you like). Serve immediately. This vegan queso is also delicious over fries, baked potatoes, and roasted vegetables.
If you have a high-powered blender such as a Vitamix or Blendtec, you can likely skip soaking the cashews all together (my KitchenAid high-powered blender has no trouble breaking them down). If you don’t have a high-powered blender but are still in a hurry, I have also read that you can grind the cashews in the blender until they resemble a fine powder (stop before they turn into cashew butter). Heat 1 1/2 cups of the broth (or water) on the stove, and then pour the hot broth into the blender and blend until smooth (beware of hot steam escaping from the top of the blender and sputtering). Proceed by adding the other ingredients.
Store cooled leftover queso in an airtight container or a bowl covered with plastic wrap in the refrigerator for up to 5 days. Gently reheat on the stove or in the microwave, stirring frequently, until warmed through.