Preheat oven to 325 degrees F. On a rimmed baking sheet, toss halved tomatoes with 2 tablespoons olive oil, sliced garlic, thyme, salt, pepper, and red pepper flakes if using. Spread in a single layer, cut sides up. Roast until tomatoes are shriveled and caramelized at the edges, 40 to 45 minutes. Let cool slightly.

Heat a large skillet or griddle over medium-high heat. Brush both sides of baguette slices with remaining olive oil. Toast until golden brown and crisp, 2 to 3 minutes per side. While still warm, rub one side of each toast with the cut side of the halved garlic clove.

Halve and pit the avocados. Scoop flesh into a bowl and add lemon juice and a pinch of salt. Mash with a fork until mostly smooth but with some chunky texture remaining.

Spread a generous layer of avocado onto each garlic-rubbed toast. Top with warm roasted tomatoes and their garlicky oil from the pan. Finish with a pinch of flaky sea salt and serve immediately.