Slice the baguette into 2" chunks. Heat a cast iron skillet over medium heat and add 2 Tbsp olive oil, along with 2 Tbsp butter - toss in a smashed garlic clove to flavor the fat. Add the cubed bread, and cook on all sides for several minutes until toasted and golden brown. Remove the garlic, and discard. Set the toasted bread aside to cool.
Preheat oven to 375 degrees. On a large sheet pan, toss the cubed butternut squash and halved Brussels sprouts with 2 Tbsp olive oil, kosher salt and black pepper. Roast for 25-30 minutes until fully cooked and golden brown on the edges. Set aside to cool.
To make the dressing, add the apple cider vinegar, both mustards, 3/4 tsp kosher salt and 1/2 tsp black pepper to a small jar, shaking to combine. Slowly add the remaining 6 Tbsp olive oil, stirring with a fork or a whisk, or shake again to emulsify. Set aside.
Optional - cover the dried cranberries with hot tap water for about 5 minutes, then drain - this softens them for texture in the salad.
When everything is relatively room temperature, toss the toasted bread, roasted vegetables, the red onion, watercress or arugula, cranberries, pistachio's, and most of the dressing into a large salad bowl. Allow to sit at room temperature for up to two hours - this also keeps well in the fridge for (slightly soggy, but delicious) leftovers the next day!