How to Make It
Straddle a 14- to 18-inch paella pan over two gas burners, and heat burners to medium-high. Add oil to pan, and swirl to coat. Once smoking, add asparagus, and season with 1/4 teaspoon salt. Cook, stirring often, until tender-crisp, 2 to 3 minutes. Transfer asparagus to a plate; set aside.
Add tomatillos, leek, and garlic, and cook, stirring often, until tomatillos are completely softened and a thin layer caramelizes on bottom of pan, adjusting heat and rotating pan as needed to prevent scorching, 7 to 10 minutes.
Pour in stock, and sprinkle with paprika, saffron, and remaining 2 teaspoons salt. Bring to a simmer over medium-high. Sprinkle rice into the simmering broth in an even layer. Make sure the rice is completely covered with liquid, and avoid stirring for the rest of the cooking process. Dot chickpeas around pan. Adjust heat to maintain a gentle simmer, and cook, rotating pan every 2 minutes or so, until rice becomes visible, about 10 minutes. Add clams, nestling hinge sides into liquid. Continue cooking undisturbed except to rotate occasionally, until rice has absorbed all liquid and you can hear crackling on bottom of pan indicating the socarrat is forming, 5 to 10 minutes. To check for socarrat, drive a small metal spoon through rice to bottom of pan and feel for a crusty, slightly sticky bottom. (It will likely form in the center of the paella first.)
Once socarrat has formed in center, position pan off-center so flame is under an edge where socarrat has yet to develop. Cook until you hear the crackling and feel the crusty bottom, then rotate again. Continue cooking and rotating until a socarrat forms on bottom of entire paella, 8 to 10 minutes.
Remove the paella from heat, and return asparagus to pan. Sprinkle with peas, and scatter with pea tendrils. Cover with aluminum foil, and let stand at least 5 minutes and up to 20 minutes. Remove and discard any clams that do not open. Drizzle generously with oil before serving.