Heat the oven to 190 C / 375 F. Lightly oil a 9x13-inch / 23x33 cm baking dish.

Slice the tops off the tomatoes and scoop out the seeds and pulp with a small spoon, leaving a 1 cm / 1/2-inch shell. Sprinkle the insides with salt and set upside down on paper towels to drain for 10 minutes.

Bring the broth to a boil in a 2-quart / 1.9 L saucepan. Add the quinoa, cover, and simmer for 15 minutes until the liquid is absorbed and the grains look fluffy. Remove from heat and let sit 5 minutes, then fluff with a fork.

While the quinoa cooks, toast the pine nuts in an 8-inch / 20 cm skillet over medium heat for 2 to 3 minutes, shaking often, until golden. Set aside.

In a mixing bowl, combine the cooked quinoa, olive oil, garlic, lemon zest, lemon juice, half the feta, parsley, mint, and black pepper. Stir until evenly mixed.

Pat the insides of the tomatoes dry with a paper towel. Spoon the quinoa filling into each tomato, packing it gently and mounding slightly at the top.

Arrange the stuffed tomatoes in the baking dish. Top each with the remaining feta and a pinch of red pepper flakes if using.

Bake for 20 to 25 minutes, until the tomato skins look wrinkled and feel tender when pressed with a spoon and the feta on top is lightly golden.

Scatter the toasted pine nuts over the tomatoes and let them cool for 5 minutes before serving.