Spread bread cubes on a baking sheet and bake, stirring once or twice, until toasted and lightly golden, about 15 minutes. Transfer baking sheet to a wire rack to cool. 

 

Using a vegetable peeler, cut 4 long strips of rhubarb lengthwise from 1 rhubarb stalk to form ribbons for tying asparagus; set aside. Cut remaining rhubarb (including peeled stalk) crosswise into 1/4 inch pieces.

 

In a large saucepan, combine chopped rhubarb, water, shallots, oil, sugar, 1 teaspoon fine sea salt and generous pinch pepper over medium-high heat. Bring to a gentle simmer and cook, stirring occasionally, until rhubarb is tender and flavors are blended, about 10 minutes. 

 

Meanwhile, in a large skillet, combine asparagus, butter and generous pinch fine sea salt; add water to just cover asparagus. Cook over high heat until asparagus are crisp-tender, about 1 minute after water comes to boil. Reserving skillet with water mixture, use tongs to transfer asparagus to a colander to drain, then transfer to a clean work surface. Add spinach to skillet; cook over medium-high heat, stirring, until wilted and tender, 1 to 2 minutes, then drain.

 

Transfer soup to a blender; carefully purée until smooth. Return soup to saucepan. Heat soup just to a simmer, then remove from heat and cover to keep warm.

 

Divide asparagus into 4 bundles; tie with reserved rhubarb ribbons. 

 

Divide soup among shallow serving bowls; divide asparagus bundles, spinach and toasted bread cubes among bowls. Drizzle with oil and sprinkle with coarse sea salt. Serve immediately.