Potato Tomato Gratin With Horseradish

Ingredients

• 4 Tbsp. extra-virgin olive oil, divided; plus more for serving (optional)
• 1 large onion, thinly sliced
• 4 garlic cloves, thinly sliced
• 2½ tsp. Diamond Crystal or 1¼ tsp. Morton kosher salt, divided
• 3 Tbsp. drained prepared horseradish
• 1 Tbsp. double-concentrated tomato paste
• ½ tsp. crushed red pepper flakes or 1 tsp. freshly ground black pepper
• 2 Tbsp. thyme leaves, divided
• 1 cup low-sodium vegetable or chicken broth or 1 tsp. Better Than Bouillon reconstituted in 1 cup water
• 1½ lb. Yukon Gold potatoes (about 4 medium), scrubbed, sliced into ¼"-thick rounds
• 1½ lb. plum tomatoes (7–8), ends trimmed, sliced into ¼"-thick rounds
• 1 (2"–3") piece fresh horseradish, peeled, or 1 oz. Parmesan, finely grated (for serving)
• Flaky sea salt

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