Sauté the leeks in 2 tablespoons butter over medium heat until they're soft, hitting them with a big four-finger pinch of salt (a teaspoon for the ladies who are measuring).
Sauté the rinsed reserved seeds in the remaining 2 teaspoons butter till golden brown and crisp, about 10 minutes over high, then low heat.
Add the squash and the dried thyme to the leeks and stir to heat through for a few minutes. Add just enough half-and-half to cover the squash. Bring to a simmer and cook on medium-low for 15 minutes or so.
Purée the squash in a blender (or in the pot with an immersion blender) till uniformly smooth.
Serve garnished as you like with yogurt, toasted seeds, and fresh thyme.