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1
Bring a large pot of water to a boil.
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2
Core tomatoes and cut a shallow X in bottom of each, then blanch tomatoes in boiling water 10 seconds. Transfer with a slotted spoon to an ice bath to stop cooking.
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3
Peel tomatoes using tip of a small paring knife, then halve tomatoes crosswise. Squeeze juice from tomato halves through a sieve into a bowl, pressing on and then discarding seeds. Finely dice tomato flesh.
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4
Bring stock, water, saffron, and tomato liquid to a simmer in a medium saucepan and keep at a bare simmer.
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5
Meanwhile, heat oil and butter in a heavy medium saucepan over medium heat until foam subsides, then add fennel and onion and cook, stirring occasionally, until softened and beginning to turn golden, 12 to 15 minutes.
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6
Add rice and cook, stirring constantly, until it turns opaque, about 3 minutes. Reduce heat to medium-low, then add wine and cook, stirring, until absorbed. Continue simmering and adding hot tomato-saffron stock, 1 cup at a time, stirring frequently and letting each addition be absorbed before adding the next, until rice is tender and creamy-looking but still al dente, 18 to 25 minutes total (you will have some stock left over).
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7
Stir in diced tomatoes, cheese, and salt and pepper to taste. Thin with stock if desired.
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8
Serve topped with chopped fennel fronds.
