BOIL THE POTATOES/SEASON THEM WHILE HOT: Add the potatoes along with a tablespoon of kosher salt and a teaspoon of ground black pepper to a large saucepan and cover with water plus an inch or two. Cover and bring to a boil over high heat. Uncover, reduce to simmer for 10 -12 minutes or until barely tender. Drain. Tip the warm potatoes back into the still warm pot and, off the heat, drizzle with 2 tablespoons olive oil and 1 tablespoon red wine vinegar. Stir in the Dijon-style mustard. Sprinkle with ½ teaspoon kosher salt and ¼ teaspoon fresh ground pepper. Stir well. Taste and adjust seasonings.
SAUTÉ THE ASPARAGUS AND MUSHROOMS: In the meantime, heat a 10-inch skillet over medium-high flame. Add 2 tablespoons olive oil and a pinch of crushed red pepper and heat 30 seconds or so. Tip in the asparagus and mushrooms; sprinkle with salt and pepper. Sauté, stirring often, for 3-4 minutes or until vegetables are nearly tender, adding garlic slices for the last minute or so. Remove from heat.
STIR THE ASPARAGUS MIXTURE, RED PEPPERS, AND SCALLIONS INTO THE POTATOES: Add the cooked asparagus mixture along with the chopped red peppers and scallions to the potatoes in the pot and stir well. Taste and adjust seasonings, including olive oil and red wine vinegar.
ARRANGE THE POTATO SALAD ON A BED OF THE SPINACH AND GARNISH: To a large platter or shallow serving bowl, add the spinach evenly and sprinkle with a little salt and pepper. Spoon the potato salad into the center of the spinach leaves. Grind a little black pepper over the entire dish and garnish with chives and chive flowers, if using. Serve warm or at room temperature.Store, well-covered, in the fridge for 3-4 days. You may want to add more olive oil, vinegar, and salt and pepper and/or let the salad come to room temperature before serving.