Toast the walnuts in a dry skillet over medium heat, stirring, until fragrant and lightly browned, 2–4 minutes; transfer to a plate to cool.
Rinse baby spinach thoroughly and spin or pat dry to remove excess moisture.
Hull and halve the strawberries; quarter large berries for even bites.
Thinly slice red onion if using and soak briefly in cold water to mellow the bite, then drain.
Crumble goat cheese or break it into small pieces with a fork.
Make the dressing by whisking together olive oil, balsamic vinegar, honey (or maple syrup), salt and pepper until emulsified.
In a large bowl, combine the spinach, strawberries and red onion; toss gently to mix.
Drizzle the dressing over the salad just before serving and toss lightly to coat the leaves evenly.
Scatter toasted walnuts and crumbled goat cheese over the dressed salad.
Taste and adjust seasoning with a pinch more salt or pepper; serve immediately to keep spinach crisp.