Chop the veggies and leave in separate piles. Mix the salt and spices in a small bowl and sprinkle evenly over all the veggies.
Place 1 tablespoon of butter in a large skillet and heat to high heat. Once the butter is melted, add the onions and garlic. Saute for 1-2 minutes until just barely soft. Remove from the skillet and add 2 tablespoons of butter. Once the butter is melted, add the mushrooms and saute' and stir until brown and cooked through, about 5-8 minutes. Remove and add the last tablespoon of butter to the skillet. Add the zucchini and summer squash to the skillet. Saute and stir for 2-3 minutes until golden, but just barely cooked through and still firm.
To serve, place a tortilla on a plate (you can double them up for extra stability) and load it with veggies. Top each taco with a generous sprinkling of crumbled panela cheese. Serve warm.