In a small saucepan, bring stock
or water to a boil. Add rice; cover and simmer on low heat 10
minutes. Transfer to a mixing bowl; cool.
In a small skillet, heat oil. Add
onion and cook, stirring frequently, until onion is translucent and
limp. Add to rice in mixing bowl.
Cut tops off tomatoes; scoop out
and discard cores and seeds with a small knife. Turn tomato cups
upside down to drain a few minutes, then turn right side up and
sprinkle to taste with salt and pepper.
Add lemon juice, chopped nuts,
parsley, and salt and pepper to rice-onion mixture. Divide evenly
among tomato shells and sprinkle each with 1 tbsp Parmesan.