For the marinated skirt steak: Mix all the spices, oil and vinegar in a small bowl. Then rub over the entire skirt steak and let the steak rest for 30 minutes.
Preheat the grill to high heat. Grill for 2 minutes per side, for medium-rare. Rest the steak under foil for at least 5 minutes. Then cut into 4 equal pieces. Turn each section and sliced thinly against the grain.
Set a large sauce pan over high heat and add the stock, onions, garlic, tomatoes, habanero and 1 teaspoon salt. Bring to a boil. Cover, reduce heat and simmer for 20 minutes.
Remove the habanero pieces and carefully pour the veggies and liquid into the blender. Open the vent on the lid a little and cover with a dish towel. Puree until smooth. *Be careful! Hot liquid in the blender can be dangerous.
Once pureed, taste for salt and heat. It may need 1/2 to 1 teaspoon more salt. If you really like spicy food, you could always puree half or all of the cooked habanero.
For slightly crispy chilaquiles, pour half the puree over the tostadas. For soft chilaquiles (like a casserole) pour all the puree over the tostadas. Allow the tostadas to sit and soak up the liquid. *For soft chilaquiles with a crispy top, bake the chilaquiles in the oven at 400 degrees, for 20-30 minutes. Garnish and serve warm with steak.