Heat the stock in a saucepan or in the microwave until simmering. Crush the saffron threads between your fingertips, place in a small bowl, cover with 1 tablespoon warm water, and set aside.

Heat the oil over medium heat in a large, heavy bottom skillet (I use cast-iron). Add the onion and cook, stirring, until tender, about 5 minutes. Add the garlic, bell peppers, and a generous pinch of salt. Continue cooking until the bell peppers begin to soften, about 3 minutes.

Add the zucchini and another pinch of salt. Cook, stirring, until the zucchini is lightly browned and softened. Add the tomato paste, paprika, and rice. Cook for 1 minute, until the grains begin to crackle. Add the tomatoes and cook until the start to break down, about 5 minutes.

Stir in the saffron with its soaking water. Season generously with salt and black pepper, then add the stock, asparagus, and artichokes. Bring to the boil and stir once. Reduce the heat to medium-low and simmer without stirring until the liquid has mostly evaporated, about 10-15 minutes.

Add the frozen peas. Continue to simmer until the rice is dry, another 5-10 minutes. At this point, a nice socarrat should have formed. Remove the paella from the heat and serve warm.