Prepare pickled onion mixture first by mixing all ingredients together. Let sit for at least 12 hours.
To prepare vinaigrette, place all ingredients except olive oil in a blender. Blend on high until incorporated. While blender is running, slowly add in olive oil until all of it is incorporated. Season with salt and pepper, and reserve.
For the salad, start by preparing squash blossoms. Mix together half of the requeson cheese with the crème fraîche, whipping until thoroughly combine. Remove the stamen from each blossom and stuff with the cheese-crème fraîche mixture. Dip each squash blossom in the tempura batter, and fry in vegetable oil until crispy. Reserve blossoms in a warm place.
On 4 chilled serving plates, spoon equal amounts of the dressing across the center of the plate. Lay one slice of each tomato and two slices of avocado on each serving plate in whatever design you choose. Top each plate with the pickled onions, the remaining requeson cheese, a squash blossom, epazote and a drizzle of the balsamic syrup.