Step 1To prepare the broccoli, remove the florets and break any large ones into bite-size pieces. With a sharp paring knife, cut off the dried-out base of the stem, then peel the tough skin, leaving the stalk in a rough squared-off log. Cut the stalk into quarter-inch dice.
Step 2Bring the water in the bottom of a steamer to a boil. Place the diced stems in the steamer basket and cook, covered, about 1 minute. Add the florets, cover again, and cook until the broccoli is bright green but still crunchy, another minute or two. Set aside.
Step 3Place the pancetta slices in the bottom of a cold skillet and place the skillet over medium-low heat. Cook until well-browned on one side, about 10 minutes, then turn and cook until the other side is browned, another 4 or 5 minutes. Do not rush the cooking or the pancetta will not be crisp. When the pancetta is crisp and well-browned, remove to a paper towel to drain. Set the drippings aside to cool.
Step 4Trim the bottoms of the mushroom stems; cut the mushrooms in quarters lengthwise.
Step 5Place the broccoli in a large bowl and add the mushrooms, green onions, blue cheese and walnuts. Crumble the pancetta over top.
Step 6Pour the pancetta drippings into a half-cup measure. Add enough oil to make one-half cup and then pour the mixture into a small bowl. Add the vinegar and the salt and whisk to emulsify.
Step 7Pour the dressing over the salad and stir to combine well. Taste and correct for seasoning with salt and vinegar and finish with a good grinding of black pepper. Refrigerate until ready to serve.