To make the candied walnuts: Place sugar and water in a small, heavy saucepan over medium-high heat. Bring to a low boil and cook until sugar dissolves, stirring until the sugar fully dissolves. Stop stirring and continue softly boiling the sugar until it turns a light caramel brown color. This will only take a couple of minutes, so keep an eye on the sugar! It will burn if you step away and forget about it.
Remove caramel from heat; carefully stir in nuts to coat evenly. Spread nuts on a baking sheet lined with a silpat or parchment paper. Use a spatula, and, working quickly, spread the walnuts evenly on the parchment or silpat. Sprinkle with 1/4 teaspoon salt. Set aside until cool and then break into small pieces.
You can make the nuts several days ahead and store in an airtight container in your fridge.
To make the salad dressing: Combine vinegar and mustard, stirring with a whisk. Gradually add oil, stirring constantly with a whisk. Whisk in a squeeze of lemon juice and taste to see if you need to adjust the seasonings.
Add both lettuces to a salad bowl and toss to combine. Top with pear and candied walnuts. Drizzle dressing evenly over salad; sprinkle with remaining 1/4 teaspoon salt and pepper. Toss gently to combine and serve immediately.