How to Make It
Preheat the oven to 350°. On a sheet of aluminum foil, drizzle the garlic heads with 2 tablespoons of the olive oil. Wrap tightly and roast for about 1 hour, until the garlic is very soft; let cool. Halve the garlic crosswise and squeeze the garlic cloves out of the skins into a small bowl. Add 3 tablespoons of the olive oil and mash to a paste; whisk in 2 tablespoons of the lemon juice and season with salt.
Meanwhile, finely grate 1 teaspoon of grapefruit zest. Using a sharp knife, cut the skin and all of the bitter white pith off of the grapefruit. Working over a medium bowl, cut in between the membranes to release the sections; discard the membranes and stir in the zest.
Heat a large cast-iron griddle with 1 tablespoon of the olive oil. Add half of the squid and cook over high heat, turning once, until golden in spots and just white throughout, about 1 minute. Transfer the squid to a large bowl. Repeat with 1 more tablespoon of the olive oil and the remaining squid.
Wipe off the griddle and add 1 tablespoon of olive oil. Add the avocado, cut side down, and cook over high heat until nicely charred, about 1 minute. Scoop out the avocado and thinly slice it.
Add the avocado, mustard greens and 1/2 cup of the roasted-garlic dressing to the squid and toss; reserve the remaining garlic dressing for another use. Season with salt. Transfer to plates.
In a small bowl, toss the radishes with the almonds and remaining 1 tablespoon each of lemon juice and olive oil; season with salt. Top the salads with the radishes and almonds and serve.