Preheat oven to 200°.
Trim the tough ends from the asparagus.
Bring a large pot of water to a boil.
Add the asparagus.
Cook, uncovered, until the asparagus is crisp-tender and still bright green, 6 to 8 minutes.
Drain and cool under running water to stop the cooking.
Put on a serving plate and sprinkle with ¼ teaspoon of the salt.
Keep warm in the oven.
Heat the oil in a skillet over medium heat.
Add the onion and cook until softened, about 3 minutes.
Add the mushrooms, cover and cook, stirring frequently, until they release their juices and are tender, 3 to 4 minutes.
Add the cumin, red pepper, and remaining ¼ teaspoon of salt, or to taste.
Remove the asparagus from the oven and spoon the mushrooms and onion on top, leaving the green tips showing.