Pre-heat oven to 400°F.
Line a large rimmed, baking sheet with parchment paper or a Silpat® baking mat. Set aside.
Combine red onion, potatoes, and tomatoes in a large glass bowl and sprinkle with basil, oregano and rosemary.
Drizzle with two tablespoons olive oil and season with salt and black pepper, to taste. Toss to coat.
Spread into a single layer onto prepared baking sheet.
Add chicken thighs to bowl and drizzle with remaining olive oil. Season with additional salt and black pepper, if desired. Turn to coat.
Nestle chicken thighs in between veggies on baking sheet.
Top each thigh with one or two lemon slices. Scatter remaining lemon slices amongst veggies on baking sheet.
Place baking sheet in pre-heated oven for 20 minutes.
Remove pan from oven and turn veggies.
Return pan to oven to roast for another 20 - 25 minutes, or until chicken is cooked through. Food Safety Tip: The safe cooking temperature for chicken is 165°F. Use an instant-read thermometer to test for doneness. Remove from oven when internal temperature at thickest part reaches at least 160°F. The temperature will continue to rise several degrees while chicken rests.
Remove from oven and let chicken rest for 5 minutes before serving.
Garnish with fresh herbs and serve immediately.