Thinly slice the red onion and place into a large bowl. Sprinkle with sea salt and add 1 Tbsp sherry vinegar and enough cold water to cover.
Using your fingers, squeeze the onion slices gently several times (this process will remove much of the onion "bite"). Drain well and set aside.
Brush any soil off the chanterelle mushrooms (quickly rinse if necessary) and slice into ¼" thick pieces.
Slice bacon crosswise into "matchsticks". Heat 2 tablespoons olive oil in a large nonstick skillet over medium-high heat.
Add the bacon; sauté briefly, stirring often, until the "matchsticks" begin to brown.
Add 2 more tablespoons olive oil, then add the chanterelles.
Add sea salt and freshly ground black pepper to taste.
Sauté until the mushrooms are just tender, about 5 minutes.
Add 1 Tbsp sherry vinegar and continue to cook, stirring frequently, until all liquid has evaporated. Remove from heat.
Crumble Blue Cheese coarsely.
Remove the lettuce leaves from the head, rinse well and drain.
Place the lettuce into a large bowl. Drizzle with remaining olive oil and toss.
Add 2 tsp sherry vinegar, season with salt and pepper, and toss again.
Arrange on salad plates. Scatter the mushrooms on top, then the crumbled Blue cheese; add the red onion and serve immediately.