Bring broth to slow simmer on a burner adjacent to where you’re cooking the risotto.
Heat the oil & 1½ tbsp. butter in a risotto pot over medium heat. Add chopped onion and sauté until tender and beginning to brown, about 10 minutes. Add garlic and stir, about 1 minute. Add tomatoes and cook, stirring occasionally, about 10 minutes.
Add rice, stir to coat. Proceed as with any risotto: Stirring fairly constantly, add broth, a half cup or so at a time, with enough time between additions for the broth to cook away/be absorbed by the rice—just to the point where the rice starts to stick (unless you’re using a non-stick pot).
Start testing for doneness when only a cup or two of the broth remain.
Finish it as you prefer—a little crunch deep in the center of each grain of rice or thoroughly cooked through, very thick or a bit loose, etc.
For the mantecare step, off the heat, stir in the 2 tbsp. butter, grated parmigiano and slivered basil leaves. [If you are feeding children averse to little bits of green in their food, serve the finely slivered basil leaves as a garnish for the adults.]
Test for salt—whether you need much more or less than about 1 teaspoon kosher depends on the saltiness of your cheese and your broth. Stir in a couple grindings fresh pepper.
Serve with extra parmigiano cheese, with basil if you’ve thus far left it out, and with a really crisp white wine to wash it down.