Put oil in a large, deep skillet over medium heat. When it’s hot, add onion or shallot and cook, stirring occasionally, until it softens, 3 to 5 minutes. Add rice and cook, stirring occasionally, until it is glossy and coated with oil, about 5 minutes. Sprinkle with salt and pepper, then stir in the squash; add the wine. Stir and let liquid bubble away.
03Begin to add the stock, about ½ cup at a time, stirring after each addition and every minute or so. When the stock is just about evaporated, add more. Keep the heat medium to medium-high and stir frequently.04When rice is just about tender and mixture is creamy, stir in shellfish or meat and continue to cook, adding more liquid if necessary, until rice is tender. The final dish should be quite moist but not soupy. Add Parmesan if you’re using it, then taste and add more salt or pepper (or both) if necessary. Garnish with basil or parsley and serve.