Preheat the oven to 450 degrees F. Line several baking sheets with parchment paper. Then chop all the vegetables. Lay the broccoli, bell peppers, and zucchini in a single layer on the baking sheets. Drizzle them lightly with olive oil and sprinkle with salt and pepper. Roast the vegetables in the oven for 10-15 minutes, until tender, but not mushy.
Meanwhile, warm the refried beans in a small sauce pot over medium heat. Then drain the sour cream so it is as firm as possible and place the sour cream, serrano pepper, lime juice, and garlic in the blender. Puree until smooth and salt to taste. Place the sliced tomatoes, corn, scallions, and cilantro in a small bowl. Sprinkle with salt and pepper, then toss.
Once the roasted vegetables are out of the oven, slather each Old El Paso tostada shell with a dollop of refried beans. Top them with the roasted vegetables, corn salsa, and a sprinkling of crumbled queso fresco. Then drizzle the Serrano Crema over the top and serve!