Combine onion, water and pinch salt in a heavy large saucepan. Bring to a simmer over medium-low heat and cook, stirring occasionally, until onion is softened, about 5 minutes. Add rice and cook, stirring constantly, until rice is translucent, about 5 minutes.

Add 1 cup broth and cook, stirring, until liquid is mostly absorbed, 5 to 7 minutes. Add ½ cup more broth and cook, stirring, until mostly absorbed. Repeat, adding liquid in ½ cupfuls, until rice is tender yet still slightly firm to the bite, and risotto is all’onda (a bit liquid, broth not fully absorbed). Stir in oil, and season to taste with salt and pepper.

Divide risotto among plates. Dollop with ricotta, sprinkle with marjoram and zest, and serve immediately.