Preheat the oven to 400F. Place baby potatoes on a baking sheet (optionally oiled). Sprinkle with salt and pepper to taste. Place in the oven for 20 min.
In the meantime, prep the asparagus. Trim asparagus, place in a single layer on a baking sheet (optionally oiled). Sprinkle with salt and pepper to taste, sprinkle with garlic powder. Set aside. When the oven timer for potatoes goes off, place asparagus in the oven as well. Set the timer for 10 minutes.
Prep and portobellos. Remove any stem pieces from the caps, scrape the inside of the caps to remove any gills, then wipe with a paper towel to remove any dirt / debris.
Combine all the marinade ingredients in a small bowl until well mixed. Brush the marinade on both sides of the portobellos and place in medium / large bowl. Pour the rest of the marinade over the mushrooms. Let sit for 5 minutes. Then flip and let sit for 5 more minutes.
While the mushrooms are marinating, make the chimichurri. Combine all the chimichurri ingredients in a small food processor or a blender and pulse until almost smooth.
In a hot grill pan or on the grill, cook the portobello caps until tender and nice grill marks show up (about 2-4 minutes per side).
Serve portobellos with potatoes and asparagus, drizzled with chimichurri sauce.