To make the vinaigrette dressing, whisk together the olive oil, vinegar, lime juice, red onion, chile, garlic, and salt. Pour the vinaigrette dressing in with the shredded beef. Add tomato and onion and stir to combine.
Spoon salpicón over tostadas and garnish with avocado.
Notes
Nutrition facts do not include the tostadas.
Can be made up to 3 days in advance (so long as you only add the avocados when you're ready to serve) and can be served cold or at room temperature.
I love using leftover brisket, but you can easily swap in flank steak, skirt steak, or any cut of beef that you can cut thinly or shred.
I've only ever had it using beef, but I suppose there would be no harm trading in shredded chicken or shredded pork instead.