Preheat oven to 375°F (190°C). Butter a 9 x 13 in (3 L) casserole
dish, and set aside. Bring a large pot of water to a boil. Cook pasta
according to package instructions, until al dente. Transfer to a
colander, rinse under cold water, and drain well.
To make topping, tear bread into pieces and pulse in a food
processor to form large crumbs. Transfer to a bowl with melted
butter, Parmesan and half the thyme. Toss to coat and set aside.
Melt butter in a large heavy saucepan over medium heat. Add
flour and cook, stirring constantly with a wooden spoon, until
bubbly but not brown, about 1 minute. Whisk in milk. Bring to a
simmer and continue to stir with a wooden spoon until thickened,
about 4 minutes. Add cheese, stirring until completely melted
and sauce is smooth, 1–2 minutes. Stir in tomatoes (with juices),
remaining thyme, salt and pepper. Add reserved pasta to pot and
stir to combine. Pour into prepared baking dish. Sprinkle evenly
with the reserved cheese, then the bread crumb mixture.
Bake until sauce is bubbling around the edges and the top is
golden, 25–30 minutes. Let stand 5 minutes before serving.