I am a huge broccoli fan and wanted to give it a flavor profile outside of my usual and obvious Italian garlic-olive oil-chile flake scenario. There's no reason to discard the broccoli stem, which has a radish-like crunch and mild flavor. Cutting it into long spears and searing it keeps the broccoli crunchy while softening its fibers a bit, and the creamy avocado sauce acts as a sort of dip. You can fork-and-knife this or eat it with your fingers.
In a medium bowl, smash avocados with a fork until chunky. Season with salt and pepper, add sour cream and buttermilk, then use a whisk to beat into a thick, slightly coarse puree. Taste the sauce and adjust seasoning as desired. Squeeze lime juice into a small bowl, then thinly slice the serrano and add it to the juice. Season with salt and stir to combine. Set avocado sauce and chile-lime mixture aside separately.2.
In a small skillet or saucepan, combine 2 tablespoons oil, sunflower seeds and coriander seeds over medium heat. Cook, swirling the pan often, until seeds are golden-brown, 1-2 minutes. Use a slotted spoon to transfer seeds to a plate; season them with salt and let cool. Save the coriander oil.3.
Trim the woody end from broccoli stalks, then peel with a vegetable peeler until you get down to the light green, tender layer. Cut the stalks lengthwise through the crown into 1/2-inch wide spears with florets and stalks attached. Some florets will break off, which is no big deal. Everything ends up in the pan.4.
Place a large cast-iron skillet over high heat and add reserved coriander oil plus remaining 3 tablespoons oil. Working in batches if needed to prevent crowding, add broccoli to pan, cut-sides down, and season with salt. Toss to coat, then cook in a single layer, undisturbed, until golden-brown on underside, 2 minutes. Turn and cook until second side is browned, 2 minutes more. You want to slightly soften the broccoli and get good color on it, which will mellow its raw flavor and assertive crunch without overcooking it. That's the goal — shoot for the very crisp end of the crisp-tender continuum.5.
Spoon avocado sauce onto a platter or big shallow bowl, then use the rounded side of the spoon to spread it out. Spoon reserved chile-lime mixture over, then place broccoli on top. Scatter seeds all over and add some dashes of hot sauce (or pass the hot sauce at the table for people to control their personal spice destinies). Put the chips out for the broccoli nay-sayers.