Make the Honey Mustard Sauce: In a medium bowl, whisk the mustard, honey, olive oil, vinegar, lemon juice, garlic, salt, and pepper to taste until smooth and well combined.

Cook the bacon: Line a baking sheet with parchment paper. Place the bacon slices in a single layer on the prepared baking sheet and transfer to the middle rack of a cold oven. Heat the oven to 425°F and cook until the bacon has reached your desired crispiness, 15 to 25 minutes. Use tongs to transfer the bacon to a cutting board, blot with a paper towel to absorb excess grease, then chop the bacon into bite-sized pieces. Set aside.

Bake the chicken and veggies: Line another large baking sheet with parchment paper. Place the chicken thighs in a large bowl, season with the salt and a few grinds of pepper and toss well to coat, then add the sweet potato and onion. Pour the Honey Mustard Sauce all over and use clean hands to thoroughly coat the chicken and veggies. Pour the chicken and potato mixture onto the prepared baking sheet and spread in an even layer.

Bake, stirring the vegetables halfway through but leaving the chicken undisturbed, until the sweet potatoes are fork-tender and a meat thermometer inserted into the thickest part of the chicken reads 165°F, 25 to 35 minutes, depending on the size of the chicken.

Make the crumble: Meanwhile, in a medium bowl, toss the arugula, pecans, feta, dates (if using), and lemon zest until well combined.

Scatter the arugula crumble mixture over the hot chicken and veggies, then top with the crispy bacon and salt and pepper to taste. Serve immediately.