Preheat the oven to 375 degrees F and place a baking sheet in the oven.
Make the Panzanella Cubes: Heat the butter in a skillet over moderate heat until it begins to brown. Add the chopped garlic and stir briefly to release its fragrance. Add the chopped thyme. Stir to combine.
Pour the butter mixture over the bread cubes in a bowl and toss briskly so butter coats cubes evenly. Season with salt and pepper. Stir in the 2 cheeses and chopped basil. Place the bread cubes in a single layer on the preheated baking sheet, scraping all the seasonings over them. Bake until lightly brown and crunchy on the outside, but still soft on the inside, about 7 to 8 minutes. Transfer to a bowl to cool.
Make the dressing: If using fresh peas, blanch them for 30 seconds in boiling water, then shock in ice water. Drain. In a stainless steel saucepan, simmer blanched peas or frozen peas and a pinch of salt in cream, until peas are tender, about 4 to 5 minutes. Transfer peas and cream to blender or food processor, add pinch of Vitamin C powder to retain bright green color, if desired. While blender or food processor is running, add chicken stock and olive oil and puree until smooth. Season with salt and pepper. Cool the puree in a bowl over an ice bath.
Combine: In a large bowl, combine the toasted bread cubes and the pea puree. Stir to coat the bread. Add the cooked peas, scallions, and pecorino Toscano.
Put the ring from a 10-inch spring form pan upside down on a large plate. Fill with the bread mixture, pressing it into place. Let stand, or refrigerate, for 30 minutes to 1 hour, then remove the ring. In a bowl, dress the spring greens with lemon juice, lemon zest and olive oil, to taste. Mound the greens lightly on top of the panzanella. Grate pecorino on top, to garnish.