Combine onion and 3 tablespoons water in a medium skillet; cook over medium-low heat until onion is softened and most water has evaporated, about 8 minutes. Add squash cubes, 1 cup water and generous pinch salt; cook, covered, at a very gentle simmer, until squash is tender, 15 to 20 minutes. Remove from heat, drain liquid and set aside vegetables.

 

Heat 1 tablespoon oil in a large heavy saucepan over medium-low heat. Add rice and cook, stirring constantly, until translucent, about 5 minutes. Add onion mixture and 3 1/2 cups packed escarole; stir to combine. 

 

Add 1 cup broth and cook, stirring, until liquid is mostly absorbed, 5 to 7 minutes. Add 1/2 cup more broth and cook, stirring, until mostly absorbed. In 1/2 cupfuls, add remaining broth, stirring until each addition is mostly absorbed before adding the next, until rice is tender yet still slightly firm to the bite (you may not use all the broth). Stir in cheese and 3 1/2 tablespoons oil. Season with salt and pepper to taste; remove from heat and set aside, covered.

 

In a large nonstick skillet, heat remaining tablespoon oil over medium-high heat. Add remaining escarole, squash strips and generous pinch salt. Cook, stirring occasionally, until vegetables are wilted and tender, about 5 minutes. 

 

Divide risotto among serving bowls; top with wilted escarole and squash. Serve immediately, with extra cheese for sprinkling.