Crisp up the corn tortillas by preheating the oven to 400°, spraying both sides of the tortillas with cooking spray, and placing them on a rimmed baking pan.
Bake for approximately 10 minutes, until crispy.
Prepare the potatoes by washing them thoroughly and dicing into bite-sized pieces.
Place the diced potatoes into a bowl and drizzle with two tablespoons of olive oil.
Add the cumin, salt and pepper, and mix well.
Place the seasoned potatoes into a grill basket and grill for approximately 35 minutes on high heat until golden brown and fork tender.
Season the beef tenderloin with steak seasoning, place on the grill and cook to medium-well.
Remove from grill, cover loosely with a tent of aluminum foil and allow to rest for 10 minutes prior to slicing into thin pieces.
Prepare the chimichurri sauce by combining all ingredients for the sauce together in a medium-sized bowl and mixing well.
To assemble the grilled steak and potato tostadas, place a crispy corn tortilla onto a plate. Pile 1/2 cup of the grilled potatoes onto the tortilla place three to four slices of the grilled steak over the potatoes.
On top of the steak add a large pinch of shredded spinach, a few slices of tomato and some slivers of onion.
Drizzle on about a tablespoon or so of the chimichurri sauce and serve.