Candied Nuts (optional step)
Line a baking sheet with parchment or wax paper. Set aside.
Melt the butter in a nonstick skillet over medium heat. Add in the nuts and brown sugar, stirring with a silicone spatula to coat the nuts in the butter and brown sugar.
Cook the mixture over medium heat, stirring constantly until the sugar begins to caramelize (about 3-4 minutes).
Spread out the candied nuts on the prepared parchment paper-lined baking sheet to cool. Make sure to press them into one layer so they can cool quickly and can be broken apart from each other.
Lemon Poppy Seed Dressing
In a medium-sized bowl, add in the olive oil, poppy seeds, Dijon mustard, fresh lemon zest, fresh lemon juice, and kosher salt. Whisk vigorously until well combined and emulsified, then allow flavors to meld at room temperature for at least 5-15 minutes.
To Serve
In a large salad bowl, toss the clean spinach, strawberry slices, the cooled candied nuts, and feta cheese together.
Serve with the lemon poppyseed dressing, but don't pour it on until ready to serve.